feat: sparser robot nudges, typed unread badge, lobby unread bump

Three owner-requested polish changes:

- robot: replace the lengthening 60-90 min -> 6 h proactive-nudge ramp with a
  flat uniform 9-12 h wait before every nudge; the existing sleep-window gate
  still skips and defers a nudge that would land in the robot's night.
- ui: colour the lobby/in-game unread dot by type -- the regular danger colour
  when a chat message is unread, a softer amber (--warn) when only nudges are.
  Adds a per-viewer unread_messages flag (chat_messages.kind='message') across
  the backend DTO, FlatBuffers wire, gateway transcode and the UI store.
- ui: float games with any unread notification to the top of the lobby's
  your-turn and opponent-turn sections (finished keeps its order), reusing the
  existing unread_chat flag.

Docs (ARCHITECTURE 7, FUNCTIONAL + _ru) updated. No DB migration; the new wire
field is backward-compatible.

pkg/fbs/scrabble.fbs

@@ -76,6 +76,10 @@ table GameView {
   // unread_chat is a per-viewer flag: the requesting player has at least one unread chat
   // entry (message or nudge) in this game. It drives the lobby and in-game unread badge.
   unread_chat:bool;
+  // unread_messages is a per-viewer flag: at least one of the unread entries is a real chat
+  // message (not just a nudge). With unread_chat it colours the badge — both set is the regular
+  // badge, unread_chat alone is the softer nudge-only badge.
+  unread_messages:bool;
 }
 
 // MoveRecord is one decoded move (a committed play, or a hint preview).

pkg/fbs/scrabblefb/GameView.go

@@ -197,8 +197,20 @@ func (rcv *GameView) MutateUnreadChat(n bool) bool {
 	return rcv._tab.MutateBoolSlot(30, n)
 }
 
+func (rcv *GameView) UnreadMessages() bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(32))
+	if o != 0 {
+		return rcv._tab.GetBool(o + rcv._tab.Pos)
+	}
+	return false
+}
+
+func (rcv *GameView) MutateUnreadMessages(n bool) bool {
+	return rcv._tab.MutateBoolSlot(32, n)
+}
+
 func GameViewStart(builder *flatbuffers.Builder) {
-	builder.StartObject(14)
+	builder.StartObject(15)
 }
 func GameViewAddId(builder *flatbuffers.Builder, id flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(id), 0)
@@ -245,6 +257,9 @@ func GameViewAddVsAi(builder *flatbuffers.Builder, vsAi bool) {
 func GameViewAddUnreadChat(builder *flatbuffers.Builder, unreadChat bool) {
 	builder.PrependBoolSlot(13, unreadChat, false)
 }
+func GameViewAddUnreadMessages(builder *flatbuffers.Builder, unreadMessages bool) {
+	builder.PrependBoolSlot(14, unreadMessages, false)
+}
 func GameViewEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }