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/gateway.ts

@@ -24,11 +24,17 @@ if (isMock && typeof window !== 'undefined') {
   // attaches a robot on a timer).
   (
     window as unknown as {
-      __mock?: { joinOpponent(): void; joinOpponentSilently(): void; adminReply(): void };
+      __mock?: {
+        joinOpponent(): void;
+        joinOpponentSilently(): void;
+        adminReply(): void;
+        setGameLimit(v: boolean): void;
+      };
     }
   ).__mock = {
     joinOpponent: () => (gateway as MockGateway).joinPendingOpponent(),
     joinOpponentSilently: () => (gateway as MockGateway).joinPendingOpponentSilently(),
     adminReply: () => (gateway as MockGateway).mockAdminReply(),
+    setGameLimit: (v: boolean) => (gateway as MockGateway).setGameLimit(v),
   };
 }