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/gen/fbs/scrabblefb/game-list.ts

@@ -33,8 +33,13 @@ gamesLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
+atGameLimit():boolean {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? !!this.bb!.readInt8(this.bb_pos + offset) : false;
+}
+
 static startGameList(builder:flatbuffers.Builder) {
-  builder.startObject(1);
+  builder.startObject(2);
 }
 
 static addGames(builder:flatbuffers.Builder, gamesOffset:flatbuffers.Offset) {
@@ -53,14 +58,19 @@ static startGamesVector(builder:flatbuffers.Builder, numElems:number) {
   builder.startVector(4, numElems, 4);
 }
 
+static addAtGameLimit(builder:flatbuffers.Builder, atGameLimit:boolean) {
+  builder.addFieldInt8(1, +atGameLimit, +false);
+}
+
 static endGameList(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createGameList(builder:flatbuffers.Builder, gamesOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createGameList(builder:flatbuffers.Builder, gamesOffset:flatbuffers.Offset, atGameLimit:boolean):flatbuffers.Offset {
   GameList.startGameList(builder);
   GameList.addGames(builder, gamesOffset);
+  GameList.addAtGameLimit(builder, atGameLimit);
   return GameList.endGameList(builder);
 }
 }