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.

docs/ARCHITECTURE.md

@@ -158,7 +158,12 @@ arrive from a platform rather than completing a mandatory registration).
   rendered in the recipient's **interface language** (`preferred_language`, en/ru), not in
   any bot-scoped language, and the friend-invite **share link** (and its caption) point at
   that one bot. First Telegram contact seeds the new account's `preferred_language` from the
-  launch `language_code` (§4); the interface language is otherwise edited in Settings.
+  launch `language_code` (§4), but the **interface language follows the device** — the system
+  guess, or an explicit Settings choice saved locally — and the bot never dictates the UI.
+  `preferred_language` is then **reconciled to the active interface locale on every session
+  adopt** (not only on a Settings change; a no-op for guests and when already equal), so the
+  server-rendered language surfaces — this push and the ad banner — always match the UI rather
+  than stranding a user who never opened Settings on the creation-time seed.
 - **Variant preferences (New Game gating).** Which variants a player may be matched into is a
   per-user **profile** setting — `variant_preferences`, a set of `engine.Variant` labels
   (`scrabble_en`, `scrabble_ru`, `erudit_ru`) edited on the Settings/Profile screen. New
@@ -640,7 +645,7 @@ in either direction (the enqueue excludes the caller's `BlockedWith` set);
   **floats games with any unread entry to the top** of the your-turn and opponent-turn
   sections (the finished section keeps its activity order). On each clear the publish-to-read
   latency is recorded; the read time itself is not retained.
-- **Profile**: `preferred_language` (en/ru, edited in Settings), display name, email
+- **Profile**: `preferred_language` (en/ru; tracks the interface language — §4), display name, email
   (confirm-code binding, see §4), **timezone**, the daily **away window**, the
   **variant preferences** (`variant_preferences`, the matchable-variant set that gates New
   Game — §3, defaulting to Erudit only, at least one enforced) and the