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/offline.svelte.ts

@@ -11,13 +11,21 @@ import type { Profile } from './model';
 
 // Not named `state` (a svelte-check hazard: `$state` would then read as a store subscription).
 let active = $state(loadOfflinePref());
+// True while the current offline state was entered automatically (no network detected), not chosen
+// by the player. An auto offline self-heals to online when the network returns; a deliberate one is
+// left as the player's choice.
+let auto = $state(false);
 
-/** offlineMode exposes the reactive deliberate-offline flag; read it in markup / $derived. */
+/** offlineMode exposes the reactive offline flags; read them in markup / $derived. */
 export const offlineMode = {
-  /** active is true while the app is in deliberate offline mode. */
+  /** active is true while the app is in offline mode (deliberate or auto-detected). */
   get active(): boolean {
     return active;
   },
+  /** auto is true while offline was entered automatically (no network), not by the player. */
+  get auto(): boolean {
+    return auto;
+  },
 };
 
 /**