feat(offline): mid-session flight-mode reactivity (auto-offline self-heals)

React to the network changing while the app is open (e.g. the player toggling
flight mode), via passive online/offline events - no polling, no battery cost:
- interface lost -> enter offline mode for the session (auto);
- interface back -> if the offline was auto, verify the gateway is really
  reachable (an interface being up does not guarantee it) and return online; a
  deliberate offline (the toggle or the cold-start dialog) is left as-is.

- offline.svelte: track `auto` (auto-detected vs the player's deliberate choice).
- connection.svelte: checkReachable is now a pure one-shot (the caller decides);
  the reachability watcher never probes in offline mode (events drive recovery).
- transport.ts: the reachability probe is exempt from the kill switch - it IS
  the mechanism that decides whether to return online, fired only deliberately.
- app.svelte.ts: initNetworkReactivity wires the events (web-only, skipped in
  the mock); called from bootstrap.

Online unaffected (skipped in the mock e2e): e2e 196. Mid-session reactivity is
contour-verified.

ui/src/lib/transport.ts

@@ -40,7 +40,9 @@ export function createTransport(baseUrl: string): GatewayClient {
   // (it must reject when there is no session, so the watcher keeps waiting rather than reporting up).
   const reachabilityProbe = async (): Promise<void> => {
     if (!token) throw new Error('no session');
-    assertOnline();
+    // Exempt from the offline kill switch: the probe IS the reachability check that decides whether to
+    // return online, and it fires only deliberately — the connection watcher is guarded off in offline
+    // mode, and checkReachable runs it on an online event — so it must reach the gateway while offline.
     await client.execute({ messageType: 'profile.get', payload: codec.empty(), requestId: '' }, { headers: headers() });
   };
   registerProbe(reachabilityProbe);