feat(lobby): cap simultaneous quick games at 10

Limit a player to 10 active quick games (auto-match + AI); friend games created
by invitation are not counted. At the cap the backend refuses both new-game
entry points — quick enqueue and invitation creation — with 409
game_limit_reached, while accepting an incoming invitation stays allowed, so
friend games are capped from the other end. The lobby disables "New Game" and
shows a low-emphasis notice, driven by a new at_game_limit flag on games.list
(no per-event payload: a turn change does not move the count, and the lobby
already re-fetches games.list on entry and every game event).

- game.MaxActiveQuickGames + Store/Service.CountActiveQuickGames (active/open
  seats, no game_invitations row; hidden games still count -> dedicated count)
- Server.ensureUnderGameLimit gating handleEnqueue + handleCreateInvitation;
  game.ErrGameLimitReached -> 409 game_limit_reached
- FB GameList.at_game_limit (regenerated Go + TS) through the gateway transcode
  and UI codec; gameListDTO + lobbycache snapshot + Lobby.svelte + i18n
- tests: integration count rule + HTTP gate + accept bypass; server error map;
  gateway transcode round-trip; UI codec + lobbycache unit; e2e gamelimit
- docs: PRERELEASE (GL), FUNCTIONAL(+ru), ARCHITECTURE 8, UI_DESIGN, backend README

ui/src/lib/codec.test.ts

@@ -241,6 +241,7 @@ describe('codec', () => {
     const games = fb.GameList.createGamesVector(b, [game]);
     fb.GameList.startGameList(b);
     fb.GameList.addGames(b, games);
+    fb.GameList.addAtGameLimit(b, true);
     b.finish(fb.GameList.endGameList(b));
 
     const gl = decodeGameList(b.asUint8Array());
@@ -250,6 +251,7 @@ describe('codec', () => {
     expect(gl.games[0].seats[0].score).toBe(13);
     expect(gl.games[0].lastActivityUnix).toBe(1717000000);
     expect(gl.games[0].vsAi).toBe(true);
+    expect(gl.atGameLimit).toBe(true);
   });
 
   it('decodes an OutgoingRequestList of account refs', () => {