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/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.',