feat(offline): offline-mode state, Settings toggle + blue chrome (Phase C)

The deliberate offline mode is now visible: an installed web-PWA user with a confirmed
email can switch to offline in Settings, and the header turns blue with an "Offline"
chip. (Network gating, the dict preload, the offline lobby + local-game creation, and the
service-worker precache follow in later Phase C steps.)

- offline.ts / offline.svelte.ts: a sticky, device-scoped reactive offline flag
  (offlineMode), distinct from connection.svelte's transient reachability signal, with the
  pure persistence + readiness helpers (offlineReady / missingDicts) unit-tested.
- Settings.svelte: an Online / Offline toggle, shown only for an installed web PWA with a
  confirmed email (the SW-launch + durable-account preconditions).
- Header.svelte: a blue-tinted nav (color-mix from the accent, so it tracks light/dark and
  a Telegram theme override) + an "Offline" chip while offline; the deliberate mode
  suppresses the transient "Connecting…" indicator.

Behaviour-preserving online (the toggle is hidden and offlineMode is false there; e2e 196
green). App entry stays within its size budget (111.6 / 112 KB gzip).

ui/src/lib/offline.svelte.ts

@@ -0,0 +1,23 @@
+// The deliberate offline MODE: a sticky, device-scoped reactive flag the app reads to gate the
+// network, tint the chrome blue and show only local games. It is the player's own choice (the
+// Settings toggle is the source of truth), distinct from connection.svelte.ts's transient
+// gateway-reachability signal. The pure persistence + readiness logic lives in offline.ts.
+
+import { loadOfflinePref, saveOfflinePref } from './offline';
+
+// Not named `state` (a svelte-check hazard: `$state` would then read as a store subscription).
+let active = $state(loadOfflinePref());
+
+/** offlineMode exposes the reactive deliberate-offline flag; read it in markup / $derived. */
+export const offlineMode = {
+  /** active is true while the app is in deliberate offline mode. */
+  get active(): boolean {
+    return active;
+  },
+};
+
+/** setOfflineMode enters or leaves offline mode and persists the choice (device-scoped). */
+export function setOfflineMode(on: boolean): void {
+  active = on;
+  saveOfflinePref(on);
+}