feat(ui): per-kind active-game limit lock on the New Game screen

Carry the caller's per-tier active-game caps and each game's kind on the
wire (Profile.game_limits + GameView.kind, additive FBS + gateway transcode
+ client codec, committed regen). The New Game screen counts the player's
active games per kind from the lobby and locks a capped start: an outline
button with a lock that opens a funnel modal instead of a game -- a sign-in
prompt for a guest, a "finish a current game first" notice for a signed-in
account (native Telegram popup, in-app modal elsewhere). The lock lifts via
the existing profile refetch after a guest->durable upgrade.

Remove the lobby's old at_game_limit New-Game tab disable + notice: the flag
(now the random-kind cap) conflicted with the per-kind lock -- it hid the
screen where the lock lives and wrongly blocked an unfulfilled kind. The New
Game tab is always enabled; the per-kind start lock is the only gate. The
at_game_limit wire field stays (unused by the client) for a later cleanup.

Tests: client lock logic + codec kind/game_limits roundtrip + gateway
transcode encode + native popup builders (unit); a mock e2e for the lock
badge and the modal.

pkg/fbs/scrabblefb/Profile.go

@@ -244,8 +244,21 @@ func (rcv *Profile) Ads(obj *AdsInfo) *AdsInfo {
 	return nil
 }
 
+func (rcv *Profile) GameLimits(obj *GameLimits) *GameLimits {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(40))
+	if o != 0 {
+		x := rcv._tab.Indirect(o + rcv._tab.Pos)
+		if obj == nil {
+			obj = new(GameLimits)
+		}
+		obj.Init(rcv._tab.Bytes, x)
+		return obj
+	}
+	return nil
+}
+
 func ProfileStart(builder *flatbuffers.Builder) {
-	builder.StartObject(18)
+	builder.StartObject(19)
 }
 func ProfileAddUserId(builder *flatbuffers.Builder, userId flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(userId), 0)
@@ -307,6 +320,9 @@ func ProfileStartDictVersionsVector(builder *flatbuffers.Builder, numElems int)
 func ProfileAddAds(builder *flatbuffers.Builder, ads flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(17, flatbuffers.UOffsetT(ads), 0)
 }
+func ProfileAddGameLimits(builder *flatbuffers.Builder, gameLimits flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(18, flatbuffers.UOffsetT(gameLimits), 0)
+}
 func ProfileEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }