Stage 8: UI social/account/history surfaces

Wire the deferred Stage 7 surfaces end-to-end (UI -> gateway transcode ->
backend REST -> existing domain services): friends (incl. one-time friend
codes), per-user blocks, friend-game invitations, profile editing + email
binding, the statistics screen, and the in-game history + GCG export.

Friends gain two add paths (interview decision, a deliberate plan change):
one-time 6-digit codes (friend_codes table, 12h TTL, single-use, rate-limited
redeem); and play-gated requests (shared game required) where an explicit
decline is permanent, an ignored request lapses after 30 days, and a code
bypasses a decline. Migration 00006 widens friendships_status_chk and adds
friend_codes.

Lobby notification badge is poll + push: a new generic `notify` event drives
it live; the client polls on open/focus. Language stays a single Settings
control that writes through to the durable account's preferred_language. GCG
export is finished-only (game.ErrGameActive) and shares/downloads the .gcg file.

Tests: backend unit + inttest (friend gate/decline/code, ListInvitations,
GetStats, GCG gate), gateway transcode round-trips + notify constructor, UI
vitest (codecs, win-rate, share choice) + Playwright social specs. Docs: PLAN
(Stage 8 done + refinements + TODO-5), ARCHITECTURE, FUNCTIONAL(+ru), UI_DESIGN,
TESTING, module READMEs.

ui/src/lib/app.svelte.ts

@@ -28,6 +28,8 @@ export const app = $state<{
   reduceMotion: boolean;
   boardLabels: BoardLabelMode;
   localeLocked: boolean;
+  /** Pending incoming friend requests + invitations, for the lobby badge. */
+  notifications: number;
 }>({
   ready: false,
   session: null,
@@ -39,6 +41,7 @@ export const app = $state<{
   reduceMotion: false,
   boardLabels: 'beginner',
   localeLocked: false,
+  notifications: 0,
 });
 
 let unsubscribeStream: (() => void) | null = null;
@@ -76,12 +79,35 @@ function openStream(): void {
         showToast(t('game.yourTurn'), 'info');
       } else if (e.kind === 'match_found') {
         navigate(`/game/${e.gameId}`);
+      } else if (e.kind === 'notify') {
+        void refreshNotifications();
       }
     },
     () => showToast(t('error.unavailable'), 'error'),
   );
 }
 
+/**
+ * refreshNotifications recomputes the lobby badge count (incoming friend requests
+ * plus open invitations). Authoritative poll, complementing the live 'notify' push.
+ * Guests have no social surfaces, so it is a no-op for them.
+ */
+export async function refreshNotifications(): Promise<void> {
+  if (!app.session || app.profile?.isGuest) {
+    app.notifications = 0;
+    return;
+  }
+  try {
+    const [incoming, invitations] = await Promise.all([
+      gateway.friendsIncoming(),
+      gateway.invitationsList(),
+    ]);
+    app.notifications = incoming.length + invitations.length;
+  } catch {
+    // Best-effort; leave the previous count on a transient failure.
+  }
+}
+
 function closeStream(): void {
   unsubscribeStream?.();
   unsubscribeStream = null;
@@ -98,6 +124,7 @@ async function adoptSession(s: Session): Promise<void> {
     handleError(err);
   }
   openStream();
+  void refreshNotifications();
 }
 
 export async function bootstrap(): Promise<void> {
@@ -186,6 +213,30 @@ export function setLocalePref(locale: Locale): void {
   app.localeLocked = true;
   setLocale(locale);
   persistPrefs();
+  void persistLanguageToServer(locale);
+}
+
+/**
+ * persistLanguageToServer writes the chosen interface language through to the
+ * durable account's preferred_language, so the single Settings control is the
+ * source of truth (guests keep only the client preference). Best-effort.
+ */
+async function persistLanguageToServer(locale: Locale): Promise<void> {
+  const p = app.profile;
+  if (!p || p.isGuest || p.preferredLanguage === locale) return;
+  try {
+    app.profile = await gateway.profileUpdate({
+      displayName: p.displayName,
+      preferredLanguage: locale,
+      timeZone: p.timeZone,
+      awayStart: p.awayStart,
+      awayEnd: p.awayEnd,
+      blockChat: p.blockChat,
+      blockFriendRequests: p.blockFriendRequests,
+    });
+  } catch {
+    // The client locale already changed; the server sync is best-effort.
+  }
 }
 
 export function setReduceMotion(on: boolean): void {
@@ -198,3 +249,11 @@ export function setBoardLabels(mode: BoardLabelMode): void {
   app.boardLabels = mode;
   persistPrefs();
 }
+
+// Refresh the lobby badge when the app returns to the foreground — a push 'notify'
+// may have been missed while the client was hidden/closed (poll + push, see §10).
+if (typeof document !== 'undefined') {
+  document.addEventListener('visibilitychange', () => {
+    if (document.visibilityState === 'visible' && app.session) void refreshNotifications();
+  });
+}