PageScene: stop overlay click handler from silencing portal'd modal clicks

Two PageScene-only divergences from how the working pages
(PageDraw / PageImage / PageVideo) handle the same upload modals:

1. `handleOverlayClick` on the top/bottom controls overlays called
   `event.stopPropagation()` for every click that bubbled through
   them. Modals rendered as JSX children of `Controls3D` (the
   upload-3D / upload-image / upload-splat dialogs) are
   React-tree descendants even though `Modal`'s `Dialog.Portal`
   puts them at `document.body` in the DOM. React synthetic
   stopPropagation calls native `Event.stopPropagation()`, which
   silences `react-drag-drop-files`'s native `addEventListener("click", u)`
   on the file-input label — so clicking the drop zone never opens
   the OS file picker.
   Narrow the handler to `event.target === event.currentTarget` so
   it only fires on the transparent overlay area itself, never on
   bubbles from children. The original "control buttons don't
   interact with canvas" intent is preserved without the side
   effect — the canvas listener is bound to the canvas element only,
   so clicks on overlay siblings never reach it regardless.

2. `tabIndex={0}` + `outline: none` on the editor canvas. Added in
   `61c4936c29` "so [the canvas] can receive keyboard focus," but
   `useViewportKeyboard` listens at `document` (not on canvas) so
   the focus isn't actually required. Other pages don't make their
   canvas focusable; remove for parity.

frontend/apps/artcraft/app/src/pages/PageScene/PageEditor.tsx

@@ -112,8 +112,15 @@ export const PageEditor = () => {
   const updateCamera = usePageSceneStore((s) => s.updateCamera);
   const deleteCamera = usePageSceneStore((s) => s.deleteCamera);
   const setSelectedCameraId = usePageSceneStore((s) => s.setSelectedCameraId);
+  // Only stops propagation for clicks on the overlay div itself (the
+  // transparent gaps between control buttons). Clicks that bubbled up
+  // from child controls pass through untouched, so React-tree-portal'd
+  // modals further down the JSX (the upload modals inside Controls3D)
+  // aren't silenced when the user clicks inside them.
   const handleOverlayClick = (event: React.MouseEvent<HTMLDivElement>) => {
-    event.stopPropagation();
+    if (event.target === event.currentTarget) {
+      event.stopPropagation();
+    }
   };
 
   useEffect(() => {

frontend/apps/artcraft/app/src/pages/PageScene/comps/EngineCanvases.tsx

@@ -17,8 +17,6 @@ export const EditorCanvas = () => {
       id="video-scene"
       width="1280px"
       height="720px"
-      tabIndex={0}
-      style={{ outline: "none" }}
     />
   );
 };