feat(ui): tile-crossword loading splash for cold lobby open

On a cold app open the lobby's game list arrives over the network; on a
slow link the empty "no games yet" line flashed before the games loaded.
Add a full-screen tile splash that lays a Scrabble crossword of ЭРУДИТ /
ЗАГРУЗКА / ОЖИДАНИЕ (Эрудит point values, hardcoded since the alphabet
table is not cached at boot) until the lobby's first load settles, then
removes itself to reveal the populated list.

- lib/splash.ts: pure layout + reveal schedule (unit-tested).
- components/Splash.svelte: App-level overlay; per-tile drop-in; loops
  ЗАГРУЗКА → ОЖИДАНИЕ until ready, dismisses on a word boundary. Static
  ЭРУДИТ under reduced motion / the mock build.
- app state: lobbyReady (set by Lobby on first settle) + splashDone,
  reset on logout.
- App.svelte: overlay while routeIsLobby && !splashDone; the plain text
  splash now only covers non-lobby deep-links during bootstrap.
- docs: UI_DESIGN + FUNCTIONAL (+ _ru).

ui/src/lib/app.svelte.ts

@@ -41,6 +41,12 @@ export interface Toast {
 
 export const app = $state<{
   ready: boolean;
+  /** Whether the lobby's first cold load has settled (success or error). The loading splash
+   *  (components/Splash.svelte) watches it to know when to dismiss; set by screens/Lobby. */
+  lobbyReady: boolean;
+  /** Whether the loading splash has finished and removed itself. Gates the App-level overlay
+   *  so a warm intra-session return to the lobby shows no splash again. */
+  splashDone: boolean;
   /** Whether the live-event stream is connected. The event hub is best-effort and never replays a
    *  missed event, so an open game waiting for an opponent uses this to recover: it polls the game
    *  state while the stream is down and refetches once on reconnect (see game/Game.svelte). */
@@ -84,6 +90,8 @@ export const app = $state<{
   resync: number;
 }>({
   ready: false,
+  lobbyReady: false,
+  splashDone: false,
   streamAlive: false,
   session: null,
   profile: null,
@@ -658,6 +666,9 @@ export async function logout(): Promise<void> {
   resetConnection();
   clearGameCache();
   clearLobby();
+  // Replay the loading splash on the next cold lobby load after a re-login.
+  app.lobbyReady = false;
+  app.splashDone = false;
   gateway.setToken(null);
   await clearSession();
   app.session = null;