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/e2e/maintenance.spec.ts

@@ -0,0 +1,22 @@
+import { expect, test } from './fixtures';
+
+// The maintenance overlay is raised in prod by the edge 503 marker (X-Scrabble-Maintenance);
+// the mock transport never produces one, so — like the offline indicator — the e2e drives it
+// through the window.__maint hook (gateway.ts, mock-only). It is app-global (mounted outside
+// the route blocks in App.svelte), so it shows without a session.
+test('maintenance overlay covers the app and lifts on recovery', async ({ page }) => {
+  await page.goto('/');
+  await expect(page.getByRole('alertdialog')).toHaveCount(0);
+
+  // A planned deploy window begins: a non-dismissable dimmed overlay covers the app.
+  await page.evaluate(() => (window as unknown as { __maint: { on(): void } }).__maint.on());
+  const overlay = page.getByRole('alertdialog');
+  await expect(overlay).toBeVisible();
+  // The retry button is present (EN "Try again" / RU "Повторить" depending on locale).
+  await expect(overlay.getByRole('button', { name: /again|Повторить/i })).toBeVisible();
+
+  // The window ends — in prod the store's poll lifts it on the first successful read; the mock
+  // has no probe, so it clears explicitly. The overlay disappears with no page reload.
+  await page.evaluate(() => (window as unknown as { __maint: { off(): void } }).__maint.off());
+  await expect(page.getByRole('alertdialog')).toHaveCount(0);
+});