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/connection.svelte.ts

@@ -27,6 +27,26 @@ export function registerProbe(fn: () => Promise<void>): void {
   probe = fn;
 }
 
+/**
+ * checkReachable runs the reachability probe once, bounded by timeoutMs, and reports whether the
+ * gateway answered — a single attempt (no retry loop), for the cold-start network decision — while
+ * updating the online signal. It reports false when no probe is registered or the timeout wins.
+ */
+export async function checkReachable(timeoutMs: number): Promise<boolean> {
+  if (!probe) return false;
+  try {
+    await Promise.race([
+      probe(),
+      new Promise<never>((_, reject) => setTimeout(() => reject(new Error('reachability timeout')), timeoutMs)),
+    ]);
+    reportOnline();
+    return true;
+  } catch {
+    reportOffline();
+    return false;
+  }
+}
+
 /** reportOnline marks the gateway reachable and stops the watcher. */
 export function reportOnline(): void {
   online = true;