feat(ui): preload ongoing games and cache the draft for an instant, jump-free game open

Opening a game from the lobby for the first time this session showed a brief
loading flash, and every open showed a two-step rack->board jump: the saved
draft (pending composition) was fetched separately and applied only after the
board had already painted the full rack.

Both stem from the full state and the draft not being available synchronously at
first paint. Cache the draft alongside view+history (CachedGame.draft), make
applyDraft take the already-fetched JSON so it runs synchronously, and fetch the
draft in the same Promise.all as state+history. setCachedGame preserves the
cached draft when the delta path omits it and clears it on a committed move
(mirroring the server). A new preload module warms the per-game cache (state,
history, draft) for the lobby's ongoing games with bounded concurrency, so
opening any of them is instant.

Tests: gamecache (preserve/clear/setCachedDraft) and preload (warm/skip) units;
existing draft-restore e2e still green.

ui/src/screens/Lobby.svelte

@@ -9,6 +9,7 @@
   import { t, type MessageKey } from '../lib/i18n/index.svelte';
   import { resultBadge } from '../lib/result';
   import { getLobby, setLobby } from '../lib/lobbycache';
+  import { preloadGames } from '../lib/preload';
   import { groupGames } from '../lib/lobbysort';
   import type { AccountRef, GameView, Invitation } from '../lib/model';
 
@@ -28,6 +29,10 @@
         app.notifications = incoming.length;
       }
       setLobby({ games, invitations, incoming });
+      // Warm the cache for the ongoing games so opening one from the lobby is instant. The list
+      // just loaded, so the connection is up; the call is non-blocking and re-running it on each
+      // lobby refresh cheaply warms any newly appeared game (already-cached ones are skipped).
+      if (connection.online) void preloadGames(games);
     } catch (e) {
       handleError(e);
     }