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/inttest/account_test.go

@@ -76,6 +76,21 @@ func TestAccountProvisionByIdentity(t *testing.T) {
 	}
 }
 
+// TestGetStatsZeroForFreshAccount checks that an account with no finished games
+// reads back the zero statistics rather than an error (the Stage 8 stats screen).
+func TestGetStatsZeroForFreshAccount(t *testing.T) {
+	ctx := context.Background()
+	store := account.NewStore(testDB)
+	id := provisionAccount(t)
+	st, err := store.GetStats(ctx, id)
+	if err != nil {
+		t.Fatalf("get stats: %v", err)
+	}
+	if (st != account.Stats{}) {
+		t.Fatalf("fresh stats = %+v, want zero", st)
+	}
+}
+
 // identityConfirmed reads the confirmed flag for one identity directly.
 func identityConfirmed(t *testing.T, kind, externalID string) bool {
 	t.Helper()