fix(i18n): reconcile preferred_language with the device's saved language

The UI language follows the device (the local choice / system guess) and is
deliberately not overridden from the account, but the advertising banner and
out-of-app push routing are resolved server-side from preferred_language. A
saved device choice the account had not recorded — picked while a guest, or
differing from the Telegram system-language seed — left the banner (and pushes)
in the wrong language until a Settings change rewrote preferred_language.

On profile load (adoptSession and the in-place link path) push the saved local
choice to the account when it differs (new pure helper languageNeedsServerSync;
no-op for guests and when already equal), so the banner and pushes match the
visible UI from the first open. Unit-tested.

ui/src/lib/app.svelte.ts

@@ -8,6 +8,7 @@ import { gateway } from './gateway';
 import { GatewayError } from './client';
 import { navigate, router } from './router.svelte';
 import { errorKey, localeFrom, setLocale, t, type Locale } from './i18n/index.svelte';
+import { languageNeedsServerSync } from './language';
 import { applyReduceMotion, applyTelegramTheme, applyTheme, type ThemePref } from './theme';
 import {
   insideTelegram,
@@ -455,6 +456,13 @@ async function adoptSession(s: Session): Promise<void> {
     // 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.
+    //
+    // 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);
   } catch (err) {
     handleError(err);
   }
@@ -475,6 +483,9 @@ 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);
 }
 
 /**
@@ -756,7 +767,7 @@ export function setLocalePref(locale: Locale): void {
  */
 async function persistLanguageToServer(locale: Locale): Promise<void> {
   const p = app.profile;
-  if (!p || p.isGuest || p.preferredLanguage === locale) return;
+  if (!p || !languageNeedsServerSync(p, locale)) return;
   try {
     app.profile = await gateway.profileUpdate({
       displayName: p.displayName,