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).

docs/UI_DESIGN.md

@@ -20,6 +20,28 @@ the game** (`growNav`) does the nav bar grow to absorb spare height, so the stri
 the title while the board and controls pin to the **bottom** for thumb reach. Every screen
 except Login uses `Screen`.
 
+## Loading splash (`components/Splash.svelte`)
+
+On a **cold app open** the lobby is the landing screen, but its game list arrives over the
+network — on a slow link the empty "no games yet" line would flash before the games load. A
+full-screen **tile splash** covers that gap: it lays a small Scrabble crossword out of the
+words **ЭРУДИТ** / **ЗАГРУЗКА** / **ОЖИДАНИЕ**, tile by tile, until the lobby's first load
+settles, then removes itself to reveal the populated list. The tiles carry their **Эрудит
+point values** (hardcoded in `lib/splash.ts`, since the alphabet table the board's
+`valueForLetter` reads is not cached yet at boot) and mirror a placed board tile's look
+(cream stock, bottom edge, drop shadow). The words form a 6×8 crossword: ЭРУДИТ horizontal,
+ЗАГРУЗКА and ОЖИДАНИЕ vertical, crossing it through the shared **Р** and **Д** (laid once).
+
+It is an **App-level overlay** shown while `routeIsLobby && !app.splashDone` (so it also
+covers the session bootstrap; a deep-link to another screen is not covered). The lobby sets
+`app.lobbyReady` when its first load settles (success **or** error), and the splash dismisses
+on the next **word boundary**. ЭРУДИТ lays over ~1 s, then the splash loops
+ЗАГРУЗКА → ОЖИДАНИЕ (clearing back to ЭРУДИТ between rounds) until ready, so even a fast load
+shows it for ~1 s. Each tile **drops in** with a brief scale + fade. Under **reduced motion**
+(or the mock build, to keep the Playwright smoke unblocked) it shows a static ЭРУДИТ and
+dismisses as soon as the lobby is ready. The pure layout and timing live in `lib/splash.ts`
+(unit-tested); `Splash.svelte` is the renderer.
+
 ## Navigation
 
 - **Back**: a thin, compact `<` drawn from two rotated CSS borders (`Header.svelte`