Simplify screenshot dimensions

Remove width/height query params from screenshot endpoint.
New sessions created by screenshot use DEFAULT_TERMINAL_SIZE.
Existing sessions keep their current size.

src/textual_webterm/local_server.py

@@ -527,26 +527,13 @@ class LocalServer:
         if route_key is None:
             raise web.HTTPNotFound(text="No running session")
 
-        # Parse requested dimensions (used when creating new sessions)
-        try:
-            req_width = int(request.query.get("width", str(DEFAULT_TERMINAL_SIZE[0])))
-        except ValueError:
-            req_width = DEFAULT_TERMINAL_SIZE[0]
-        req_width = max(10, min(400, req_width))
-
-        try:
-            req_height = int(request.query.get("height", str(DEFAULT_TERMINAL_SIZE[1])))
-        except ValueError:
-            req_height = DEFAULT_TERMINAL_SIZE[1]
-        req_height = max(5, min(200, req_height))
-
         session_process = self.session_manager.get_session_by_route_key(RouteKey(route_key))
         if session_process is None and route_key in self.session_manager.apps_by_slug:
-            # Create session with requested dimensions
+            # Create session with default dimensions
             await self._create_terminal_session(
                 route_key,
-                width=req_width,
+                width=DEFAULT_TERMINAL_SIZE[0],
-                height=req_height,
+                height=DEFAULT_TERMINAL_SIZE[1],
             )
             session_process = self.session_manager.get_session_by_route_key(RouteKey(route_key))
             # Give the session a moment to start and produce initial output