Fix frozen client on ctrl+q: send bye instead of half-closing

Dropping the client's channel sender never closed the socket (the
server's reader thread keeps the connection open), so the client froze
waiting for frames. The server now sends a bye message; the client
exits cleanly on it and its socket close lets the server clean up.
Messages from already-disconnected clients are ignored. Verified
end-to-end with the real client in a PTY (exits 0, daemon survives).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/integration.py

@@ -148,13 +148,15 @@ def main():
         disp2.recv_until("frame")
         disp2.send(key_event(ch="q", modifiers="CONTROL"))
         try:
+            got_bye = False
             deadline = time.time() + 3
             while time.time() < deadline:
-                disp2.recv()
-            disconnected = False
+                if disp2.recv().get("type") == "bye":
+                    got_bye = True
+                    break
         except (EOFError, TimeoutError, OSError):
-            disconnected = True
-        check("ctrl+q från klient kopplar ner klienten", disconnected)
+            pass  # EOF duger också som nedkoppling
+        check("ctrl+q från klient ger bye till klienten", got_bye)
         frame = disp.recv_until("frame")
         check("daemonen överlever klientens exit", frame.get("width", 0) > 0)