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/internal/notify/notify_test.go

@@ -98,3 +98,15 @@ func TestChatMessagePayloadRoundTrips(t *testing.T) {
 		t.Fatalf("decoded wrong chat message: %+v", ev)
 	}
 }
+
+func TestNotificationPayloadRoundTrips(t *testing.T) {
+	uid := uuid.New()
+	in := notify.Notification(uid, notify.NotifyFriendRequest)
+	if in.UserID != uid || in.Kind != notify.KindNotification || in.EventID == "" {
+		t.Fatalf("intent metadata wrong: %+v", in)
+	}
+	ev := fb.GetRootAsNotificationEvent(in.Payload, 0)
+	if got := string(ev.Kind()); got != notify.NotifyFriendRequest {
+		t.Fatalf("notification sub-kind = %q, want %q", got, notify.NotifyFriendRequest)
+	}
+}