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/components/MaintenanceOverlay.svelte

@@ -0,0 +1,83 @@
+<script lang="ts">
+  // Non-dismissable maintenance cover. Shown while the edge is in a planned deploy window
+  // (maintenance.svelte.ts, raised from a caddy 503 carrying X-Scrabble-Maintenance). It has
+  // no close affordance — an in-session user cannot dismiss it — but the store's poll lifts
+  // it automatically the moment the gateway answers again; the "retry" button just forces an
+  // immediate re-check. Mirrors the static caddy maintenance page (deploy/caddy/maintenance.html)
+  // so a fresh load and an in-session user see the same thing.
+  import { maintenance, retryNow } from '../lib/maintenance.svelte';
+  import { t } from '../lib/i18n/index.svelte';
+</script>
+
+{#if maintenance.active}
+  <div class="scrim" role="alertdialog" aria-modal="true" aria-labelledby="maint-title" aria-describedby="maint-body">
+    <div class="card">
+      <div class="tile" aria-hidden="true">Э</div>
+      <h1 id="maint-title">{t('maintenance.title')}</h1>
+      <p id="maint-body">{t('maintenance.body')}</p>
+      <button type="button" onclick={retryNow}>{t('maintenance.retry')}</button>
+    </div>
+  </div>
+{/if}
+
+<style>
+  .scrim {
+    position: fixed;
+    inset: 0;
+    /* Above the game (z 60) and toasts (z 50) — a planned window covers everything the user
+       could otherwise interact with; below the dev DebugPanel (z 10000). */
+    z-index: 100;
+    background: rgba(0, 0, 0, 0.55);
+    display: grid;
+    place-items: center;
+    padding: 24px;
+    box-sizing: border-box;
+  }
+  .card {
+    max-width: 22rem;
+    text-align: center;
+    background: var(--surface);
+    color: var(--text);
+    border-radius: var(--radius);
+    box-shadow: var(--shadow);
+    padding: 2rem 1.5rem;
+  }
+  /* Mirrors a placed board tile (as Splash.svelte / the caddy page do). */
+  .tile {
+    display: inline-grid;
+    place-items: center;
+    width: 3.5rem;
+    height: 3.5rem;
+    font-size: 2rem;
+    font-weight: 700;
+    border-radius: 4px;
+    background: var(--tile-bg);
+    color: var(--tile-text);
+    box-shadow:
+      inset 0 -2px 0 var(--tile-edge),
+      2px 0 3px -1px rgba(0, 0, 0, 0.4);
+    margin-bottom: 1.25rem;
+  }
+  h1 {
+    font-size: 1.25rem;
+    margin: 0 0 0.5rem;
+  }
+  p {
+    margin: 0 0 1.5rem;
+    color: var(--text-muted);
+    line-height: 1.5;
+  }
+  button {
+    font: inherit;
+    padding: 0.55rem 1.4rem;
+    border: 1px solid var(--border);
+    border-radius: var(--radius);
+    background: transparent;
+    color: var(--text);
+    cursor: pointer;
+  }
+  button:hover {
+    border-color: var(--accent);
+    color: var(--accent);
+  }
+</style>