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/social/social.go

@@ -19,11 +19,15 @@ import (
 )
 
 // GameReader is the slice of the game domain the social package needs: the seated
-// accounts in seat order, the seat index whose turn it is, and the game status.
-// game.Service satisfies it, so chat and nudge gate on game state without a
-// dependency on the engine or the game's private state.
+// accounts in seat order, the seat index whose turn it is, and the game status, plus
+// a shared-game test. game.Service satisfies it, so chat, nudge and the
+// befriend-an-opponent gate work without a dependency on the engine or the game's
+// private state.
 type GameReader interface {
 	Participants(ctx context.Context, gameID uuid.UUID) (seats []uuid.UUID, toMove int, status string, err error)
+	// SharedGame reports whether two accounts are seated together in any game
+	// (active or finished); it gates the "befriend an opponent" request path.
+	SharedGame(ctx context.Context, a, b uuid.UUID) (bool, error)
 }
 
 // Sentinel errors returned by the service.
@@ -38,6 +42,16 @@ var (
 	ErrRequestBlocked = errors.New("social: the addressee is not accepting friend requests")
 	// ErrRequestNotFound is returned when no pending friend request matches.
 	ErrRequestNotFound = errors.New("social: no pending friend request")
+	// ErrNoSharedGame is returned when a friend request targets someone the
+	// requester has never shared a game with (the befriend-an-opponent gate).
+	ErrNoSharedGame = errors.New("social: you can only request someone you have played with")
+	// ErrRequestDeclined is returned when the addressee has previously declined a
+	// request from this requester; a re-send is refused (a one-time friend code
+	// from the addressee bypasses this).
+	ErrRequestDeclined = errors.New("social: this person has declined your friend request")
+	// ErrFriendCodeInvalid is returned when a redeemed friend code is unknown,
+	// already used, or expired.
+	ErrFriendCodeInvalid = errors.New("social: friend code is invalid or expired")
 	// ErrNotParticipant is returned when an account is not seated in the game.
 	ErrNotParticipant = errors.New("social: account is not a player in this game")
 	// ErrChatBlocked is returned when the sender has disabled chat for themselves.