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/App.svelte

@@ -6,6 +6,7 @@
   import { t } from './lib/i18n/index.svelte';
   import { insideTelegram, telegramBackButton } from './lib/telegram';
   import Toast from './components/Toast.svelte';
+  import Splash from './components/Splash.svelte';
   import StaleInviteModal from './components/StaleInviteModal.svelte';
   import WelcomeRedeemModal from './components/WelcomeRedeemModal.svelte';
   import Login from './screens/Login.svelte';
@@ -22,6 +23,11 @@
     void bootstrap();
   });
 
+  // The lobby is the cold-start landing (an empty hash and Telegram launch params both parse
+  // to it). The tile loading splash overlays it until its first load settles; a deep-link to
+  // another screen is not covered.
+  const routeIsLobby = $derived(router.route.name === 'lobby');
+
   // Inside Telegram, drive its native header back button: show it on any sub-screen
   // (everything returns to the lobby root), hide it on the lobby/login. The app's own
   // back chevron is hidden in Telegram (Header.svelte) so only the native one shows.
@@ -72,7 +78,11 @@
 </script>
 
 {#if !app.ready}
-  <div class="splash">{t('common.loading')}</div>
+  <!-- On a cold lobby open the tile splash (below) covers bootstrap too; a deep-link to
+       another screen keeps the plain text splash while bootstrap runs. -->
+  {#if !routeIsLobby}
+    <div class="splash">{t('common.loading')}</div>
+  {/if}
 {:else if app.blocked}
   <Blocked />
 {:else}
@@ -113,6 +123,10 @@
 <StaleInviteModal />
 <WelcomeRedeemModal />
 
+{#if routeIsLobby && !app.splashDone && !app.blocked}
+  <Splash />
+{/if}
+
 <style>
   .splash {
     height: 100%;