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/i18n/en.ts

@@ -214,6 +214,9 @@ export const en = {
   'settings.labelsClassic': 'Classic',
   'settings.labelsNone': 'None',
   'settings.reduceMotion': 'Reduce motion',
+  'settings.offlineMode': 'Play mode',
+  'settings.online': 'Online',
+  'settings.offline': 'Offline',
 
   'about.title': 'About',
   'about.tab': 'Info',

ui/src/lib/i18n/ru.ts

@@ -214,6 +214,9 @@ export const ru: Record<MessageKey, string> = {
   'settings.labelsClassic': 'Классика',
   'settings.labelsNone': 'Без текста',
   'settings.reduceMotion': 'Меньше анимаций',
+  'settings.offlineMode': 'Режим игры',
+  'settings.online': 'Онлайн',
+  'settings.offline': 'Оффлайн',
 
   'about.title': 'О программе',
   'about.tab': 'Инфо',

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);
+}

ui/src/lib/offline.test.ts

@@ -0,0 +1,38 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { loadOfflinePref, saveOfflinePref, offlineReady, missingDicts } from './offline';
+import type { Variant } from './model';
+
+// A minimal in-memory localStorage for the persistence tests (node has none).
+beforeEach(() => {
+  const store = new Map<string, string>();
+  (globalThis as unknown as { localStorage: Storage }).localStorage = {
+    getItem: (k: string) => store.get(k) ?? null,
+    setItem: (k: string, v: string) => void store.set(k, v),
+    removeItem: (k: string) => void store.delete(k),
+    clear: () => store.clear(),
+    key: () => null,
+    length: 0,
+  } as Storage;
+});
+
+describe('offline mode helpers', () => {
+  it('persists and reads the device-scoped offline flag', () => {
+    expect(loadOfflinePref()).toBe(false);
+    saveOfflinePref(true);
+    expect(loadOfflinePref()).toBe(true);
+    saveOfflinePref(false);
+    expect(loadOfflinePref()).toBe(false);
+  });
+
+  it('offlineReady requires every enabled variant to have a dictionary', () => {
+    const has = (v: Variant): boolean => v !== 'erudit_ru';
+    expect(offlineReady(['scrabble_en', 'scrabble_ru'], has)).toBe(true);
+    expect(offlineReady(['scrabble_en', 'erudit_ru'], has)).toBe(false);
+    expect(offlineReady([], has)).toBe(false);
+  });
+
+  it('missingDicts lists the enabled variants without a dictionary', () => {
+    const has = (v: Variant): boolean => v === 'scrabble_en';
+    expect(missingDicts(['scrabble_en', 'scrabble_ru', 'erudit_ru'], has)).toEqual(['scrabble_ru', 'erudit_ru']);
+  });
+});

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));
+}