Stage 17 round 6 (#16/#17, PR C): lobby sort + server-derived in-game friend state

Lobby: group the my-games list into your-turn / opponent-turn / finished
(empty sections hidden), ordered by last activity (your-turn oldest-first,
the other two newest-first), as a compact line-separated list. gameDTO and
FB GameView gain last_activity_unix (turn start while active, finish time
once finished); a pure lib/lobbysort.ts holds the grouping/ordering.

Friends: the in-game 'add to friends' item is now server-derived via a new
GET /user/friends/outgoing (+ friends.outgoing op), returning addressees with
a pending OR declined request (both read as 'request sent'), so it is correct
across reloads; it shows a disabled '✓ in friends' once accepted. It
live-updates when the opponent answers: RespondFriendRequest now publishes
friend_added (accept) / friend_declined (new notify sub-kind, decline) to the
original requester, whose open game re-derives its friend state.

Tests: lobbysort unit test; gateway outgoing + last_activity transcode tests;
backend integration ListOutgoingRequests + respond-publishes-to-requester;
e2e updated for the new lobby section labels + a non-friend active opponent.
Docs: ARCHITECTURE notify catalog, FUNCTIONAL(+ru) lobby/friends, PLAN.

pkg/fbs/scrabblefb/GameView.go

@@ -149,8 +149,20 @@ func (rcv *GameView) SeatsLength() int {
 	return 0
 }
 
+func (rcv *GameView) LastActivityUnix() int64 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(24))
+	if o != 0 {
+		return rcv._tab.GetInt64(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *GameView) MutateLastActivityUnix(n int64) bool {
+	return rcv._tab.MutateInt64Slot(24, n)
+}
+
 func GameViewStart(builder *flatbuffers.Builder) {
-	builder.StartObject(10)
+	builder.StartObject(11)
 }
 func GameViewAddId(builder *flatbuffers.Builder, id flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(id), 0)
@@ -185,6 +197,9 @@ func GameViewAddSeats(builder *flatbuffers.Builder, seats flatbuffers.UOffsetT)
 func GameViewStartSeatsVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
 	return builder.StartVector(4, numElems, 4)
 }
+func GameViewAddLastActivityUnix(builder *flatbuffers.Builder, lastActivityUnix int64) {
+	builder.PrependInt64Slot(10, lastActivityUnix, 0)
+}
 func GameViewEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }