Stage 17: UI defect fixes (russian variant, Telegram theme/nav/banner, reconnect, hint zoom, plaque, history, transitions, per-game cache)

- #6 align the UI variant id to the backend canonical 'russian_scrabble' (type, variants, Lobby, mock, tests) — fixes the New->Russian 400
- #11/#12 inside Telegram force the colour scheme from WebApp.colorScheme (over OS prefers-color-scheme, fixing the Telegram Desktop breakage) and hide the theme switcher
- #14/#15 nav bar takes Telegram's bg; announcement banner gets a dedicated subtle --ad-bg accent token
- #16 suppress the reconnect banner while backgrounded and silently reconnect the live stream on return to the foreground
- #17 hint zoom scrolls to the placement's bounding box, not the top-left
- #19/#20 players plaque: active seat raised with side shadows, others sunk; tap toggles history
- #21/#23 history: scrollbar-gutter:stable (no word jitter); fixed-height drawer pins the bottom shadow to the board
- #3 (UI) disable nudge on the player's own turn
- #18a directional screen slide transitions (forward in from the right, back reveals the lobby)
- #13 per-game in-memory cache: instant render on re-entry + background refresh
- e2e: openGame waits for the slide transition to settle

ui/src/lib/theme.ts

@@ -27,6 +27,7 @@ export interface TelegramThemeParams {
   button_color?: string;
   button_text_color?: string;
   secondary_bg_color?: string;
+  header_bg_color?: string;
 }
 
 /** applyTelegramTheme overrides token values at runtime from Telegram themeParams. */
@@ -44,4 +45,8 @@ export function applyTelegramTheme(p: TelegramThemeParams): void {
   set(p.button_color, '--accent');
   set(p.button_text_color, '--accent-text');
   set(p.link_color, '--accent');
+  // The nav bar tracks Telegram's chrome so it doesn't fall out of the design; the
+  // announcement banner takes the secondary surface so it reads as a subtle accent.
+  set(p.header_bg_color ?? p.bg_color, '--bg-elev');
+  set(p.secondary_bg_color, '--ad-bg');
 }