Stage 7 polish: app shell + nav + lobby + settings (Parts A/B/C)

- Screen.svelte shell: nav bar grows, ad+content+tabbar pinned bottom (mobile feel)
- AdBanner.svelte + banner.ts rotator (params, mock long/short, linkify); Header CSS chevron + grow; Menu (bigger CSS hamburger); TabBar + HoldConfirm shared components; user-select:none
- Lobby: hide-empty sections, tab order New/Tournaments/Stats, place-based result badges (result.ts)
- Settings: Board style > Labels (beginner/classic/none) + prefs plumbing (boardlabels.ts); i18n keys + ru mirror

ui/src/lib/session.ts

@@ -6,6 +6,7 @@
 import type { Session } from './model';
 import type { ThemePref } from './theme';
 import type { Locale } from './i18n/catalog';
+import type { BoardLabelMode } from './boardlabels';
 
 const DB_NAME = 'scrabble';
 const STORE = 'kv';
@@ -122,6 +123,7 @@ export interface Prefs {
   theme: ThemePref;
   locale: Locale;
   reduceMotion: boolean;
+  boardLabels: BoardLabelMode;
 }
 
 export async function loadPrefs(): Promise<Partial<Prefs>> {