Stage 8: UI social/account/history surfaces

Wire the deferred Stage 7 surfaces end-to-end (UI -> gateway transcode ->
backend REST -> existing domain services): friends (incl. one-time friend
codes), per-user blocks, friend-game invitations, profile editing + email
binding, the statistics screen, and the in-game history + GCG export.

Friends gain two add paths (interview decision, a deliberate plan change):
one-time 6-digit codes (friend_codes table, 12h TTL, single-use, rate-limited
redeem); and play-gated requests (shared game required) where an explicit
decline is permanent, an ignored request lapses after 30 days, and a code
bypasses a decline. Migration 00006 widens friendships_status_chk and adds
friend_codes.

Lobby notification badge is poll + push: a new generic `notify` event drives
it live; the client polls on open/focus. Language stays a single Settings
control that writes through to the durable account's preferred_language. GCG
export is finished-only (game.ErrGameActive) and shares/downloads the .gcg file.

Tests: backend unit + inttest (friend gate/decline/code, ListInvitations,
GetStats, GCG gate), gateway transcode round-trips + notify constructor, UI
vitest (codecs, win-rate, share choice) + Playwright social specs. Docs: PLAN
(Stage 8 done + refinements + TODO-5), ARCHITECTURE, FUNCTIONAL(+ru), UI_DESIGN,
TESTING, module READMEs.

backend/cmd/backend/main.go

@@ -141,6 +141,7 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	matchmaker.SetNotifier(hub)
 	go matchmaker.RunReaper(ctx, cfg.Lobby.ReaperInterval)
 	invitations := lobby.NewInvitationService(lobby.NewStore(db), games, accounts, socialSvc)
+	invitations.SetNotifier(hub)
 	logger.Info("lobby and social domains ready", zap.Duration("robot_wait", cfg.Lobby.RobotWait))
 
 	srv := server.New(cfg.HTTPAddr, server.Deps{