Middle mouse button only moves the camera

A press with the middle (or any other non-left) mouse button never reaches
the Studio's drag, place or select handlers: dragging pans the view even
on a selected object or while placing, and a click selects nothing. A
middle click no longer starts the browser's autoscroll.

check_studio covers a middle-drag on the selected truck (it stays put,
the view moves), a middle click on another object (the selection is
kept), and a middle-drag while placing (nothing is placed).

studio/index.html

@@ -70,7 +70,7 @@ document.documentElement.setAttribute("data-theme",t);})();
     <tr><th>Turn</th><td>R / Shift+R, or the ⟲ ⟳ buttons</td></tr>
     <tr><th>Copy / delete</th><td>Ctrl+D / Delete</td></tr>
     <tr><th>Undo / redo</th><td>Ctrl+Z / Ctrl+Y</td></tr>
-    <tr><th>View</th><td>drag the ground to pan, Ctrl + wheel or pinch to zoom, ↺ ↻ to rotate, levels to look inside</td></tr>
+    <tr><th>View</th><td>drag the ground to pan, or drag with the middle mouse button anywhere (it never moves an object); Ctrl + wheel or pinch to zoom, ↺ ↻ to rotate, levels to look inside</td></tr>
     <tr><th>Agent</th><td>✦ Agent opens a chat with a language model that edits the scene for you; Undo takes its steps back. Drag the window by its title, resize it at the top-left corner</td></tr>
     <tr><th>MCP</th><td>connect an AI agent over MCP; its edits appear here live (see the panel when nothing is selected)</td></tr>
   </table>