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/events.go

@@ -83,6 +83,19 @@ func MatchFound(userID, gameID uuid.UUID) Intent {
 	return Intent{UserID: userID, Kind: KindMatchFound, Payload: b.FinishedBytes(), EventID: eventID()}
 }
 
+// Notification is a lightweight "re-poll" signal to userID that a friend request or
+// invitation changed. kind is a sub-discriminator (NotifyFriendRequest,
+// NotifyFriendAdded, NotifyInvitation, NotifyGameStarted) the client may use to
+// scope its refresh.
+func Notification(userID uuid.UUID, kind string) Intent {
+	b := flatbuffers.NewBuilder(32)
+	k := b.CreateString(kind)
+	fb.NotificationEventStart(b)
+	fb.NotificationEventAddKind(b, k)
+	b.Finish(fb.NotificationEventEnd(b))
+	return Intent{UserID: userID, Kind: KindNotification, Payload: b.FinishedBytes(), EventID: eventID()}
+}
+
 // eventID returns a best-effort correlation id for one emitted event.
 func eventID() string {
 	if id, err := uuid.NewV7(); err == nil {