Add web server error handling and proper shutdown

- Add stop() method to WebServer class
- Store server instance for graceful shutdown
- Add error handling in start_server() with user-friendly alert
- Show warning dialog if server fails to start (e.g., port in use)
- Stop server in closeEvent before application exit
- Add status bar messages for server state

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

web_server.py

@@ -254,8 +254,13 @@ class WebServer:
             port=self.port,
             log_level="warning"
         )
-        server = uvicorn.Server(config)
-        server.run()
+        self.server = uvicorn.Server(config)
+        self.server.run()
+
+    def stop(self):
+        """Stop the web server."""
+        if self.server:
+            self.server.should_exit = True
 
     async def run_async(self):
         """Run the web server asynchronously."""