feat(ui): in-session maintenance overlay on the edge 503 marker

The edge caddy 503 carries X-Scrabble-Maintenance during a deploy window, but a user
already in the running SPA only sees their calls start failing — the static caddy page
catches only a fresh load. Detect that marker (strictly — not any transient
'unavailable', which the Connecting indicator already covers) on the raw ConnectError at
the two transport catch sites (unary exec + the live subscribe stream), before
toGatewayError discards the response headers, and raise a non-dismissable dimmed overlay
that mirrors the static caddy page. It self-clears: a capped-backoff poll (a cheap read,
mirroring connection.svelte.ts) lifts it on the first success, and a manual "retry"
button forces an immediate re-check — so it can never get stuck. On detection the read
retry loop fails fast, so a window doesn't burn the retry budget on every call.

- pure detector maintenance.ts (maintenanceRetryMs / parseRetryAfterMs) + unit tests;
  the store + self-clearing poll in maintenance.svelte.ts (mirrors connection.svelte.ts)
- MaintenanceOverlay.svelte (clones Splash's fixed/inset/dimmed shell, non-dismissable),
  mounted app-global in App.svelte after Coachmark
- transport.ts detects + reports at both catch sites, clears on any successful read
- i18n RU "Технические работы" / EN "Under maintenance"; a window.__maint mock hook
  (the mock can't emit a real 503) + a Playwright spec

Prod is same-origin (VITE_GATEWAY_URL empty) so the marker header is readable without a
CORS expose-header. Verified: pnpm check (0), unit (402), build, e2e (186, Chromium+WebKit).

ui/src/lib/gateway.ts

@@ -6,6 +6,7 @@ import type { GatewayClient } from './client';
 import { MockGateway } from './mock/client';
 import { createTransport } from './transport';
 import { reportOffline, reportOnline } from './connection.svelte';
+import { clearMaintenance, reportMaintenance } from './maintenance.svelte';
 
 const isMock = import.meta.env.MODE === 'mock';
 
@@ -20,6 +21,12 @@ if (isMock && typeof window !== 'undefined') {
     offline: reportOffline,
     online: reportOnline,
   };
+  // The mock never produces a real 503, so the e2e drives the maintenance overlay directly
+  // (the store's poll is inert in mock — no probe is registered — so `off` clears it).
+  (window as unknown as { __maint?: { on(retryAfterMs?: number): void; off(): void } }).__maint = {
+    on: (retryAfterMs = 15000) => reportMaintenance(retryAfterMs),
+    off: clearMaintenance,
+  };
   // Drive the auto-match opponent join deterministically from the e2e (the mock otherwise
   // attaches a robot on a timer).
   (

ui/src/lib/i18n/en.ts

@@ -7,6 +7,9 @@ export const en = {
   'app.brand': 'Erudit',
   'dict.loading': 'Loading…',
   'connection.connecting': 'Connecting…',
+  'maintenance.title': 'Under maintenance',
+  'maintenance.body': 'Scrabble is briefly down for an update. It will be back in a moment — no need to reload the page.',
+  'maintenance.retry': 'Try again',
 
   'blocked.title': 'Account blocked',
   'blocked.permanent': 'Your account is blocked.',

ui/src/lib/i18n/ru.ts

@@ -8,6 +8,9 @@ export const ru: Record<MessageKey, string> = {
   'app.brand': 'Эрудит',
   'dict.loading': 'Загрузка…',
   'connection.connecting': 'Подключение…',
+  'maintenance.title': 'Технические работы',
+  'maintenance.body': 'Идёт короткое обновление игры. Мы скоро вернёмся — страницу перезагружать не нужно.',
+  'maintenance.retry': 'Повторить',
 
   'blocked.title': 'Учётная запись заблокирована',
   'blocked.permanent': 'Ваша учётная запись заблокирована.',

ui/src/lib/maintenance.svelte.ts

@@ -0,0 +1,83 @@
+// Global maintenance signal + self-clearing poll. `active` is true while the edge is in a
+// planned deploy window (a caddy 503 carried `X-Scrabble-Maintenance`; see maintenance.ts).
+// A non-dismissable overlay (MaintenanceOverlay.svelte) covers the app while active, and a
+// cheap read is retried on a capped-backoff cadence (mirroring connection.svelte.ts) — the
+// first success lifts the overlay, so it can never get stuck even with no other traffic.
+// Mock mode never reports maintenance, so it simply stays inactive.
+
+import { backoffMs } from './retry';
+
+let active = $state(false);
+let retryHintMs = $state(0);
+let pollTimer: ReturnType<typeof setTimeout> | null = null;
+let attempt = 0;
+let probing = false;
+let probe: (() => Promise<void>) | null = null;
+
+export const maintenance = {
+  /** active is true while the edge is in a planned maintenance window. */
+  get active(): boolean {
+    return active;
+  },
+  /** retryHintMs is the last Retry-After hint in milliseconds (0 when unknown). */
+  get retryHintMs(): number {
+    return retryHintMs;
+  },
+};
+
+/** registerMaintenanceProbe installs the reachability read the poll fires while active
+ *  (the transport wires a cheap authenticated read). */
+export function registerMaintenanceProbe(fn: () => Promise<void>): void {
+  probe = fn;
+}
+
+/** reportMaintenance raises the overlay and starts the self-clearing poll. Idempotent: a
+ *  repeat hit only refreshes the hint, it never restarts a running poll. */
+export function reportMaintenance(retryAfterMs: number): void {
+  retryHintMs = retryAfterMs;
+  if (active) return;
+  active = true;
+  attempt = 0;
+  if (probe) schedulePoll();
+}
+
+/** clearMaintenance lowers the overlay and stops the poll (a successful read, or reset). */
+export function clearMaintenance(): void {
+  active = false;
+  if (pollTimer) {
+    clearTimeout(pollTimer);
+    pollTimer = null;
+  }
+}
+
+/** retryNow forces an immediate re-check (the overlay's button). It never closes the
+ *  overlay itself — only a successful probe does. A no-op while a probe is in flight. */
+export function retryNow(): void {
+  if (!active || !probe || probing) return;
+  if (pollTimer) {
+    clearTimeout(pollTimer);
+    pollTimer = null;
+  }
+  attempt = 0;
+  runProbe();
+}
+
+function schedulePoll(): void {
+  pollTimer = setTimeout(runProbe, backoffMs(++attempt));
+}
+
+function runProbe(): void {
+  pollTimer = null;
+  if (!active || !probe || probing) return;
+  probing = true;
+  probe().then(
+    () => {
+      probing = false;
+      clearMaintenance();
+    },
+    () => {
+      probing = false;
+      if (active) schedulePoll();
+    },
+  );
+}

ui/src/lib/maintenance.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, it } from 'vitest';
+import { Code, ConnectError } from '@connectrpc/connect';
+import { maintenanceRetryMs, parseRetryAfterMs } from './maintenance';
+
+// A raw ConnectError as connect-web builds it from a non-Connect HTTP 503: the response
+// headers are preserved as `.metadata` (verified against @connectrpc/connect-web).
+const err503 = (headers: Record<string, string>): ConnectError =>
+  new ConnectError('HTTP 503', Code.Unavailable, new Headers(headers));
+
+describe('maintenanceRetryMs', () => {
+  it('detects the marker and parses Retry-After', () => {
+    expect(maintenanceRetryMs(err503({ 'x-scrabble-maintenance': '1', 'retry-after': '120' }))).toBe(120_000);
+  });
+
+  it('matches the header case-insensitively, defaulting the delay when Retry-After is absent', () => {
+    expect(maintenanceRetryMs(err503({ 'X-Scrabble-Maintenance': '1' }))).toBe(15_000);
+  });
+
+  it('returns null for a 503 without the marker (a plain transient failure)', () => {
+    expect(maintenanceRetryMs(err503({ 'retry-after': '30' }))).toBeNull();
+  });
+
+  it('returns null for non-Connect errors', () => {
+    expect(maintenanceRetryMs(new Error('boom'))).toBeNull();
+    expect(maintenanceRetryMs(undefined)).toBeNull();
+    expect(maintenanceRetryMs('nope')).toBeNull();
+  });
+});
+
+describe('parseRetryAfterMs', () => {
+  it('clamps into the 3s–120s band', () => {
+    expect(parseRetryAfterMs('120')).toBe(120_000);
+    expect(parseRetryAfterMs('1')).toBe(3_000); // floor
+    expect(parseRetryAfterMs('9999')).toBe(120_000); // ceiling
+  });
+
+  it('falls back to the default on absent / garbage / non-positive', () => {
+    expect(parseRetryAfterMs(null)).toBe(15_000);
+    expect(parseRetryAfterMs(undefined)).toBe(15_000);
+    expect(parseRetryAfterMs('soon')).toBe(15_000);
+    expect(parseRetryAfterMs('0')).toBe(15_000);
+    expect(parseRetryAfterMs('-5')).toBe(15_000);
+  });
+});

ui/src/lib/maintenance.ts

@@ -0,0 +1,37 @@
+// Detection of a planned edge maintenance window. During a prod deploy roll the edge
+// caddy returns HTTP 503 with an `X-Scrabble-Maintenance: 1` header (and a `Retry-After`)
+// for every gated route. The SPA keys STRICTLY on that marker — not on any transport
+// `unavailable`, which the "Connecting…" indicator already covers — so a planned window
+// raises the maintenance overlay while an ordinary blip stays a soft reconnect.
+//
+// These helpers are pure (no DOM, no runes) so they unit-test in the node vitest env. The
+// maintenance marker + Retry-After ride on the raw ConnectError's `metadata`, which
+// toGatewayError (retry.ts) discards — so detection must run on the raw error.
+
+import { ConnectError } from '@connectrpc/connect';
+
+const DEFAULT_RETRY_MS = 15_000;
+const MIN_RETRY_MS = 3_000;
+const MAX_RETRY_MS = 120_000;
+
+/**
+ * parseRetryAfterMs converts a `Retry-After` header (delta-seconds — the edge emits
+ * seconds, never an HTTP-date) into a millisecond hint clamped to a sane band. An absent
+ * or unparseable value falls back to a default.
+ */
+export function parseRetryAfterMs(header: string | null | undefined): number {
+  const secs = header == null ? NaN : Number(header);
+  if (!Number.isFinite(secs) || secs <= 0) return DEFAULT_RETRY_MS;
+  return Math.min(MAX_RETRY_MS, Math.max(MIN_RETRY_MS, Math.round(secs * 1000)));
+}
+
+/**
+ * maintenanceRetryMs returns the maintenance `Retry-After` hint in milliseconds when the
+ * thrown transport error is the edge maintenance 503 (carries `X-Scrabble-Maintenance: 1`),
+ * or null for any other error. Header lookup is case-insensitive (Headers.get).
+ */
+export function maintenanceRetryMs(e: unknown): number | null {
+  if (!(e instanceof ConnectError)) return null;
+  if (e.metadata?.get('x-scrabble-maintenance') !== '1') return null;
+  return parseRetryAfterMs(e.metadata.get('retry-after'));
+}

ui/src/lib/transport.ts

@@ -12,6 +12,8 @@ import { GatewayError, type GatewayClient } from './client';
 import * as codec from './codec';
 import { browserOffset } from './profileValidation';
 import { registerProbe, reportOffline, reportOnline } from './connection.svelte';
+import { clearMaintenance, registerMaintenanceProbe, reportMaintenance } from './maintenance.svelte';
+import { maintenanceRetryMs } from './maintenance';
 import { backoffMs, isConnectionCode, retryable, toGatewayError } from './retry';
 
 const MAX_RETRIES = 6;
@@ -28,10 +30,14 @@ export function createTransport(baseUrl: string): GatewayClient {
 
   // The reachability probe the connection watcher fires while offline: a cheap authenticated read
   // (it must reject when there is no session, so the watcher keeps waiting rather than reporting up).
-  registerProbe(async () => {
+  const reachabilityProbe = async (): Promise<void> => {
     if (!token) throw new Error('no session');
     await client.execute({ messageType: 'profile.get', payload: codec.empty(), requestId: '' }, { headers: headers() });
-  });
+  };
+  registerProbe(reachabilityProbe);
+  // The maintenance overlay reuses the same cheap read to poll for the end of a deploy
+  // window; the first success lifts the overlay (maintenance.svelte.ts).
+  registerMaintenanceProbe(reachabilityProbe);
 
   // exec runs one unary op, auto-retrying transient transport failures with capped backoff (so a
   // dropped connection or a rate-limit recovers seamlessly) and driving the global Connecting
@@ -43,6 +49,14 @@ export function createTransport(baseUrl: string): GatewayClient {
         res = await client.execute({ messageType, payload, requestId: '' }, { headers: headers(), signal });
       } catch (e) {
         if (signal?.aborted) throw e; // an intentional cancel (e.g. the tiles moved) — do not retry
+        const maintMs = maintenanceRetryMs(e);
+        if (maintMs !== null) {
+          // A planned deploy window (the edge 503 carried X-Scrabble-Maintenance): raise the
+          // overlay and fail fast — its own poll drives recovery — instead of burning the
+          // retry budget on every read for the length of the window.
+          reportMaintenance(maintMs);
+          throw toGatewayError(e);
+        }
         const err = toGatewayError(e);
         if (retryable(err.code, messageType) && attempt < MAX_RETRIES) {
           reportOffline();
@@ -53,6 +67,7 @@ export function createTransport(baseUrl: string): GatewayClient {
         throw err;
       }
       reportOnline();
+      clearMaintenance();
       if (res.resultCode && res.resultCode !== 'ok') throw new GatewayError(res.resultCode);
       return res.payload;
     }
@@ -310,7 +325,11 @@ export function createTransport(baseUrl: string): GatewayClient {
             if (pe) onEvent(pe);
           }
         } catch (e) {
-          if (!ctrl.signal.aborted) onError?.(toGatewayError(e));
+          if (!ctrl.signal.aborted) {
+            const maintMs = maintenanceRetryMs(e);
+            if (maintMs !== null) reportMaintenance(maintMs);
+            onError?.(toGatewayError(e));
+          }
         }
       })();
       return () => ctrl.abort();