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/connection.svelte.ts

@@ -9,6 +9,7 @@
 // other traffic is in flight.
 
 import { backoffMs } from './retry';
+import { offlineMode } from './offline.svelte';
 
 let online = $state(true);
 let watchTimer: ReturnType<typeof setTimeout> | null = null;
@@ -39,10 +40,8 @@ export async function checkReachable(timeoutMs: number): Promise<boolean> {
       probe(),
       new Promise<never>((_, reject) => setTimeout(() => reject(new Error('reachability timeout')), timeoutMs)),
     ]);
-    reportOnline();
     return true;
   } catch {
-    reportOffline();
     return false;
   }
 }
@@ -71,7 +70,9 @@ function scheduleProbe(attempt: number): void {
   watchTimer = setTimeout(
     () => {
       watchTimer = null;
-      if (online || !probe) return;
+      // Never probe the network in offline mode (the kill switch); the online/offline events drive
+      // recovery there instead.
+      if (online || !probe || offlineMode.active) return;
       probe().then(reportOnline, () => scheduleProbe(Math.min(attempt + 1, 6)));
     },
     backoffMs(attempt),