feat(offline): auto-detect no network at cold start + 'go offline?' dialog

A sticky-online cold start with no network hung the splash on adoptSession's
retrying profile fetch. Now, for an offline-capable web install with a cached
profile:
- No network interface (navigator.onLine === false) -> enter offline mode for
  the session (no dialog; the next launch re-evaluates).
- Interface up but the gateway is unreachable within 3s (a single-attempt
  reachability probe, not the 6-retry loop) -> a 'No connection. Enable offline
  mode?' dialog: Enable -> sticky offline; Keep trying -> the normal online
  adopt (retries, 'Connecting...').

- connection.svelte: checkReachable(timeout) - a bounded single probe.
- offline.svelte: setOfflineMode(on, persist) - auto-offline is session-only, a
  deliberate choice (dialog/toggle) is sticky.
- app.svelte.ts: the cold-start auto-detect in bootstrap + the dialog resolver;
  App.svelte renders the boot dialog. i18n en/ru.
- App-entry bundle budget 113->114 (the boot path cannot be lazy-loaded).

Online cold-start unaffected (auto-detect gated to isStandalone, off in the mock
e2e): e2e 196. The offline paths are contour-verified.

Next: PR2 - mid-session flight-mode reactivity (online/offline events).

ui/src/lib/i18n/en.ts

@@ -218,6 +218,9 @@ export const en = {
   'settings.online': 'Online',
   'settings.offline': 'Offline',
   'offline.preloadWarning': 'Poor internet connection. Some features may be unavailable.',
+  'offline.promptTitle': 'No connection. Enable offline mode?',
+  'offline.promptYes': 'Enable',
+  'offline.promptNo': 'Keep trying',
 
   'about.title': 'About',
   'about.tab': 'Info',