fix(rooms): harden chat-room parsing and TUI state from review findings

Adversarial review of the chat-rooms feature surfaced seven confirmed
issues; all fixed with regression tests:

lib (DoS hardening — a malicious/MITM server could send a tiny frame
with a ~4-billion element count):
- room_list: bound the name/count vector loops by remaining payload.
- join_room: bound the member-list loop the same way.

TUI (soulseek-rs/src/models/rooms.rs, ui):
- a message for a room we already left no longer resurrects its tab
  (route via open_index, not ensure_open).
- closing a tab now clears the unread badge on the room we land on.
- a room-list refresh keeps the highlight on the same room across a
  re-sort (track selection by name, not index).
- Esc in the room list clears a lingering filter before closing, matching
  the title's 'Esc: clear' hint.
- chat log renders one row per line so the newest message is never
  clipped by wrapping.

soulseek-rs-lib/src/message/server/join_room.rs

@@ -19,6 +19,11 @@ impl MessageHandler<ServerMessage> for JoinRoomHandler {
         let user_count = message.read_int32();
         let mut users = Vec::new();
         for _ in 0..user_count {
+            // Guard against a hostile user_count outrunning the payload; each
+            // username is a length-prefixed string (>= 4 bytes).
+            if message.get_pointer() + 4 > message.get_size() {
+                break;
+            }
             users.push(message.read_string());
         }
         let _ = sender.send(ServerMessage::RoomJoined { room, users });
@@ -29,6 +34,26 @@ impl MessageHandler<ServerMessage> for JoinRoomHandler {
 mod tests {
     use super::*;
 
+    #[test]
+    fn hostile_user_count_does_not_hang() {
+        // room="" then user_count=u32::MAX with no usernames: must return
+        // promptly instead of looping ~4 billion times.
+        let (tx, rx) = std::sync::mpsc::channel();
+        let mut message = Message::new();
+        message.write_raw_bytes(vec![0u8; 8]);
+        message.write_string("");
+        message.write_int32(u32::MAX);
+        message.set_pointer(8);
+
+        JoinRoomHandler.handle(&mut message, tx);
+        match rx.try_recv() {
+            Ok(ServerMessage::RoomJoined { users, .. }) => {
+                assert!(users.is_empty());
+            }
+            other => panic!("unexpected: {other:?}"),
+        }
+    }
+
     #[test]
     fn forwards_room_and_member_list() {
         let (tx, rx) = std::sync::mpsc::channel();