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.ts

@@ -0,0 +1,43 @@
+// Pure helpers for the deliberate offline MODE — its device-scoped persistence and the readiness
+// decision — kept out of the reactive module (offline.svelte.ts) so they unit-test in the node env.
+// The deliberate offline mode is distinct from connection.svelte.ts's transient "can we reach the
+// gateway" signal: it is the player's own sticky choice, and it gates the network, tints the chrome
+// and shows only local games.
+
+import type { Variant } from './model';
+
+const STORAGE_KEY = 'scrabble.offlineMode';
+
+/** loadOfflinePref reads the persisted offline-mode flag (device-scoped); false when unset or when
+ *  storage is unavailable, so a device that cannot persist simply starts online. */
+export function loadOfflinePref(): boolean {
+  try {
+    return typeof localStorage !== 'undefined' && localStorage.getItem(STORAGE_KEY) === '1';
+  } catch {
+    return false;
+  }
+}
+
+/** saveOfflinePref persists the offline-mode flag (best-effort). */
+export function saveOfflinePref(on: boolean): void {
+  try {
+    if (typeof localStorage !== 'undefined') localStorage.setItem(STORAGE_KEY, on ? '1' : '0');
+  } catch {
+    /* best-effort — a failed persist just reverts to online on the next launch */
+  }
+}
+
+/**
+ * offlineReady reports whether the device can play offline right now: at least one variant is
+ * enabled and every enabled variant's dictionary is already available on the device (hasDict). The
+ * offline toggle uses it to decide whether flipping to offline can succeed immediately.
+ */
+export function offlineReady(enabled: readonly Variant[], hasDict: (v: Variant) => boolean): boolean {
+  return enabled.length > 0 && enabled.every((v) => hasDict(v));
+}
+
+/** missingDicts lists the enabled variants whose dictionary is not yet available — the ones the
+ *  toggle must fetch (or wait on) before offline mode can be entered. */
+export function missingDicts(enabled: readonly Variant[], hasDict: (v: Variant) => boolean): Variant[] {
+  return enabled.filter((v) => !hasDict(v));
+}