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/gamecache.ts

@@ -0,0 +1,30 @@
+// In-memory per-game cache. A game the player has opened once is kept here so a
+// later re-entry renders instantly from the cache while a fresh fetch updates it in
+// the background — removing the blank "loading" flash and the full redraw on every
+// lobby <-> game navigation. It is intentionally process-memory only (no persistence):
+// stale entries are corrected by the background refresh, and the cache is cleared on
+// logout.
+
+import type { MoveRecord, StateView } from './model';
+
+interface CachedGame {
+  view: StateView;
+  moves: MoveRecord[];
+}
+
+const cache = new Map<string, CachedGame>();
+
+/** getCachedGame returns the last-seen state+history for a game, or undefined. */
+export function getCachedGame(id: string): CachedGame | undefined {
+  return cache.get(id);
+}
+
+/** setCachedGame stores the latest state+history for a game. */
+export function setCachedGame(id: string, view: StateView, moves: MoveRecord[]): void {
+  cache.set(id, { view, moves });
+}
+
+/** clearGameCache drops every cached game (called on logout). */
+export function clearGameCache(): void {
+  cache.clear();
+}