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/i18n/en.ts

@@ -33,6 +33,7 @@ export const en = {
   'lobby.finishedGames': 'Finished games',
   'lobby.noActive': 'No active games yet.',
   'lobby.noFinished': 'No finished games yet.',
+  'lobby.limitReached': "You've reached the simultaneous games limit.",
   'lobby.new': 'New',
   'lobby.stats': 'Stats',
   'lobby.profile': 'Profile',

ui/src/lib/i18n/ru.ts

@@ -34,6 +34,7 @@ export const ru: Record<MessageKey, string> = {
   'lobby.finishedGames': 'Завершённые игры',
   'lobby.noActive': 'Пока нет активных игр.',
   'lobby.noFinished': 'Пока нет завершённых игр.',
+  'lobby.limitReached': 'Вы достигли лимита одновременных партий',
   'lobby.new': 'Новая',
   'lobby.stats': 'Статы',
   'lobby.profile': 'Профиль',