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/offline.svelte.ts

@@ -20,10 +20,15 @@ export const offlineMode = {
   },
 };
 
-/** setOfflineMode enters or leaves offline mode and persists the choice (device-scoped). */
-export function setOfflineMode(on: boolean): void {
+/**
+ * setOfflineMode enters or leaves offline mode. By default it persists the choice (device-scoped) —
+ * a deliberate choice (the Settings toggle, or the cold-start "no connection" dialog). Pass
+ * persist=false for a transient, auto-detected offline (a cold start with no network interface): the
+ * flag holds for the session but is not saved, so the next launch re-evaluates the network.
+ */
+export function setOfflineMode(on: boolean, persist = true): void {
   active = on;
-  saveOfflinePref(on);
+  if (persist) saveOfflinePref(on);
 }
 
 // The dict-preload warning: true when a first-lobby background preload could not fetch every