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.

pkg/fbs/scrabblefb/InvitationInvitee.go

@@ -0,0 +1,97 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package scrabblefb
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type InvitationInvitee struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsInvitationInvitee(buf []byte, offset flatbuffers.UOffsetT) *InvitationInvitee {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &InvitationInvitee{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishInvitationInviteeBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsInvitationInvitee(buf []byte, offset flatbuffers.UOffsetT) *InvitationInvitee {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &InvitationInvitee{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedInvitationInviteeBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *InvitationInvitee) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *InvitationInvitee) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *InvitationInvitee) AccountId() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *InvitationInvitee) DisplayName() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *InvitationInvitee) Seat() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *InvitationInvitee) MutateSeat(n int32) bool {
+	return rcv._tab.MutateInt32Slot(8, n)
+}
+
+func (rcv *InvitationInvitee) Response() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(10))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func InvitationInviteeStart(builder *flatbuffers.Builder) {
+	builder.StartObject(4)
+}
+func InvitationInviteeAddAccountId(builder *flatbuffers.Builder, accountId flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(accountId), 0)
+}
+func InvitationInviteeAddDisplayName(builder *flatbuffers.Builder, displayName flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(displayName), 0)
+}
+func InvitationInviteeAddSeat(builder *flatbuffers.Builder, seat int32) {
+	builder.PrependInt32Slot(2, seat, 0)
+}
+func InvitationInviteeAddResponse(builder *flatbuffers.Builder, response flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(3, flatbuffers.UOffsetT(response), 0)
+}
+func InvitationInviteeEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}