fix(i18n): reconcile preferred_language to the interface locale on every adopt

A user who never changed the language in Settings kept their account at the
creation-time preferred_language seed (e.g. en from the Telegram launch language_code)
even after switching the device to another language: the UI followed the device (ru) but
the ad banner and out-of-app push — both resolved server-side from preferred_language —
stayed en. The on-adopt reconcile was gated on an explicit local choice (localeLocked),
so a system-guess locale was never pushed through.

Reconcile preferred_language to the active interface locale (app.locale) on every session
adopt and link, regardless of how the locale was chosen; persistLanguageToServer already
self-gates (a no-op for guests and when already equal), so there is no steady-state write.
The banner and push are the only server-rendered language surfaces and both read
preferred_language, so this keeps the whole interface consistent — not just the banner.
Drop the now-dead localeLocked flag (the reconcile guards were its only readers; the saved
prefs.locale still restores the UI choice per device).

Trade-off: preferred_language now follows the most-recently-opened device, so an explicit
choice on one device can be overwritten by a system guess on another (the "explicit" mark
is local, per-device); making it globally sticky would need a DB flag.

Docs: ARCHITECTURE §4 + the profile field.

ui/src/lib/app.svelte.ts

@@ -68,7 +68,6 @@ export const app = $state<{
   locale: Locale;
   reduceMotion: boolean;
   boardLabels: BoardLabelMode;
-  localeLocked: boolean;
   /** Pending incoming friend requests, for the lobby ⚙️ badge and the Settings Friends tab. */
   notifications: number;
   /** Per-game flag: the player has at least one unread chat entry (message or nudge) in that
@@ -109,7 +108,6 @@ export const app = $state<{
   locale: 'en',
   reduceMotion: false,
   boardLabels: 'beginner',
-  localeLocked: false,
   notifications: 0,
   chatUnread: {},
   messageUnread: {},
@@ -451,18 +449,20 @@ async function adoptSession(s: Session): Promise<void> {
   await saveSession(s);
   try {
     app.profile = await gateway.profileGet();
-    // The live interface language follows the device — the explicit local choice (locked, saved
-    // in prefs) or the system guess made at bootstrap — and is no longer overridden from the
-    // account here. preferred_language stays the user's saved choice (written from Settings,
-    // and used for out-of-app push routing), but the Telegram bot a user signs in through must
-    // not dictate the UI: a ru-bot launch on an English system stays English.
+    // The live interface language follows the device — the explicit local choice (saved in
+    // prefs) or the system guess made at bootstrap — and is no longer overridden from the
+    // account here: the Telegram bot a user signs in through must not dictate the UI, so a
+    // ru-bot launch on an English system stays English.
     //
-    // But the banner and out-of-app push routing ARE resolved from preferred_language, so an
-    // explicit device choice the account has not recorded yet (picked while a guest, or
-    // differing from the Telegram system-language seed) would otherwise leave them in the wrong
-    // language until the next Settings change. Reconcile the account to the saved local choice
-    // here; persistLanguageToServer no-ops for guests and when already equal.
-    if (app.localeLocked) void persistLanguageToServer(app.locale);
+    // The banner and out-of-app push are resolved server-side from preferred_language, so it
+    // must track whatever language the UI actually shows — the explicit choice AND the system
+    // guess. Reconcile it to the active locale on every adopt, not only after an explicit
+    // Settings choice: a user who never opened Settings would otherwise be stuck on the
+    // creation-time seed — e.g. an English banner under a Russian UI. This keeps every
+    // server-rendered, language-dependent surface (banner, out-of-app push) aligned with the
+    // interface, not just one. persistLanguageToServer self-gates (a no-op for guests and when
+    // already equal), so there is no write in the steady state.
+    void persistLanguageToServer(app.locale);
   } catch (err) {
     handleError(err);
   }
@@ -483,9 +483,10 @@ export async function applyLinkResult(r: LinkResult): Promise<void> {
     return;
   }
   app.profile = await gateway.profileGet();
-  // A guest who chose a language and then linked in place now has a durable account: push the
-  // saved choice so the banner + push routing follow it (see adoptSession).
-  if (app.localeLocked) void persistLanguageToServer(app.locale);
+  // A guest who linked in place now has a durable account: push the active interface language
+  // so the banner + push routing follow it (see adoptSession — reconciled regardless of an
+  // explicit Settings choice).
+  void persistLanguageToServer(app.locale);
 }
 
 /**
@@ -538,7 +539,6 @@ export async function bootstrap(): Promise<void> {
   applyReduceMotion(app.reduceMotion);
   if (prefs.locale) {
     app.locale = prefs.locale;
-    app.localeLocked = true;
     setLocale(prefs.locale);
   } else {
     const guess = localeFrom(typeof navigator !== 'undefined' ? navigator.language : 'en');
@@ -754,7 +754,6 @@ export function setTheme(theme: ThemePref): void {
 
 export function setLocalePref(locale: Locale): void {
   app.locale = locale;
-  app.localeLocked = true;
   setLocale(locale);
   persistPrefs();
   void persistLanguageToServer(locale);