Stage 11: account linking & merge (email + Telegram Login Widget)

Link an email (confirm-code) or Telegram (web Login Widget) to the current
account; if the identity already has its own account, merge the two into the
one in use (the current account is primary, except a guest initiator whose
durable counterpart wins). The merge runs in one transaction
(internal/accountmerge): stats + hint wallet summed, paid_account ORed,
identities/games/chat/complaints transferred, friends/blocks de-duplicated,
the secondary kept as a merged_into tombstone so a shared finished game's
no-cascade FKs hold; a shared active game blocks the merge.

- migration 00009: accounts.paid_account, merged_into, merged_at (+ jetgen)
- internal/link orchestrator; session.RevokeAllForAccount on merge
- connector ValidateLoginWidget RPC + loginwidget HMAC validator
- edge ops link.email.request/confirm/merge, link.telegram.confirm/merge;
  supersedes the Stage 8 email.bind.* surface (request never reveals 'taken'
  before the code is verified, so a probe cannot enumerate addresses)
- UI Profile link section + irreversible-merge dialog; Telegram web sign-in
- focused regression tests (merge core, guest inversion, active-game refusal,
  finished-shared-game kept), gateway transcode + connector + UI codec/e2e
- docs: PLAN, ARCHITECTURE 3/4/9, FUNCTIONAL(+ru), module READMEs

gateway/internal/transcode/encode.go

@@ -61,6 +61,40 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 	return b.FinishedBytes()
 }
 
+// encodeLinkResult builds a LinkResult payload (Stage 11). A switched-session token
+// (a guest initiator whose durable counterpart won) is carried as a nested Session
+// for the client to adopt; it is omitted otherwise.
+func encodeLinkResult(r backendclient.LinkResultResp) []byte {
+	b := flatbuffers.NewBuilder(256)
+	status := b.CreateString(r.Status)
+	secID := b.CreateString(r.SecondaryUserID)
+	secName := b.CreateString(r.SecondaryName)
+	hasSession := r.Token != "" && r.Profile != nil
+	var sess flatbuffers.UOffsetT
+	if hasSession {
+		token := b.CreateString(r.Token)
+		uid := b.CreateString(r.Profile.UserID)
+		name := b.CreateString(r.Profile.DisplayName)
+		fb.SessionStart(b)
+		fb.SessionAddToken(b, token)
+		fb.SessionAddUserId(b, uid)
+		fb.SessionAddIsGuest(b, r.Profile.IsGuest)
+		fb.SessionAddDisplayName(b, name)
+		sess = fb.SessionEnd(b)
+	}
+	fb.LinkResultStart(b)
+	fb.LinkResultAddStatus(b, status)
+	fb.LinkResultAddSecondaryUserId(b, secID)
+	fb.LinkResultAddSecondaryDisplayName(b, secName)
+	fb.LinkResultAddSecondaryGames(b, int32(r.SecondaryGames))
+	fb.LinkResultAddSecondaryFriends(b, int32(r.SecondaryFriends))
+	if hasSession {
+		fb.LinkResultAddSession(b, sess)
+	}
+	b.Finish(fb.LinkResultEnd(b))
+	return b.FinishedBytes()
+}
+
 // encodeMoveResult builds a MoveResult payload.
 func encodeMoveResult(r backendclient.MoveResultResp) []byte {
 	b := flatbuffers.NewBuilder(512)