fix(offline): set the auto flag in setOfflineMode; remove temp diagnostic

The auto-offline -> online return was dead because setOfflineMode never set the
'auto' flag: I added the state + getter but forgot the assignment, so it stayed
false. So scheduleRecovery bailed immediately (no poll ran) and the online event's
'if (active && auto)' was false. The contour diagnostic confirmed offline.auto
stayed false after an auto-offline. Add 'auto = on && !persist'.

Also remove the temporary network diagnostic panel (netdiag + the lobby strip) --
it did its job of pinpointing this.

ui/src/lib/offline.svelte.ts

@@ -36,6 +36,7 @@ export const offlineMode = {
  */
 export function setOfflineMode(on: boolean, persist = true): void {
   active = on;
+  auto = on && !persist; // a non-persisted offline is auto-detected; a persisted one is deliberate
   if (persist) saveOfflinePref(on);
 }