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.

docs/ARCHITECTURE.md

@@ -585,7 +585,10 @@ and **invitation-update** carry the full invitation, and the client upserts a st
 drops a terminal one (started, declined, cancelled, expired) — the invitations list is a delta channel
 too, fresh from any screen without a refetch. The move-commit **response** (`submit_play` / `pass` /
 `exchange` / `resign`) likewise returns the actor's own refilled rack and bag size, so the mover
-renders the next turn without a self-refetch. The `notify` package owns the FlatBuffers encoding
+renders the next turn without a self-refetch. Beyond that event-driven warming, the lobby
+**preloads** the player's ongoing games — each one's `game.state`, `game.history` and saved
+**draft** — into that same per-game cache, so opening one from the lobby is instant and a saved
+composition paints already on the board (no rack→board step). The `notify` package owns the FlatBuffers encoding
 (fed wire-agnostic input structs by the domain services) and the gateway forwards every payload
 verbatim. Auto-match needs no match poll — `Enqueue` returns the game the player enters
 synchronously, and an opponent later taking the open seat arrives as the in-app **opponent-joined**