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/server/handlers_game.go

@@ -243,6 +243,32 @@ func (s *Server) handleHistory(c *gin.Context) {
 	c.JSON(http.StatusOK, historyDTO{GameID: gameID.String(), Moves: moves})
 }
 
+// gcgDTO is a game's GCG export: a suggested filename plus the GCG text.
+type gcgDTO struct {
+	GameID   string `json:"game_id"`
+	Filename string `json:"filename"`
+	Content  string `json:"content"`
+}
+
+// handleExportGCG returns a finished game's GCG transcript for download/share. The
+// service refuses an active game (ErrGameActive) to avoid leaking the live journal.
+func (s *Server) handleExportGCG(c *gin.Context) {
+	_, gameID, ok := s.userGame(c)
+	if !ok {
+		return
+	}
+	gcg, err := s.games.ExportGCG(c.Request.Context(), gameID)
+	if err != nil {
+		s.abortErr(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, gcgDTO{
+		GameID:   gameID.String(),
+		Filename: "game-" + gameID.String() + ".gcg",
+		Content:  gcg,
+	})
+}
+
 // handleListGames returns the caller's active and finished games for the lobby.
 func (s *Server) handleListGames(c *gin.Context) {
 	uid, ok := userID(c)