fix(shell): clip native_surface overlay to its panel's corner radius

The web canvas is a GPUI `native_surface` element, which on macOS
attaches an `NSView` (with a Metal-backed `CALayer`) as an AppKit
overlay on top of the GPUI render layer. GPUI's `overflow: hidden`
clips its own children but cannot reach into AppKit, so the canvas
painted square corners on top of the main panel's rounded edge —
the white surface visibly overshot the panel's rounded bottom.

Thread a per-corner radius through the GPUI patch:

- `Window::sync_native_surface` and the `PlatformWindow` trait carry
  a `Corners<Pixels>`.
- The macOS implementation sets `CALayer.cornerRadius` to the maximum
  requested radius and `maskedCorners` to the bitmask of corners that
  actually requested rounding (GPUI is Y-down while NSView is Y-up,
  so a GPUI "top" maps to a CALayer "MaxY" corner — comment locks
  the mapping). `setMasksToBounds: YES` lets Core Animation actually
  honor the corner radius for layered content.
- The `NativeSurface` element resolves all four corner radii from its
  own `StyleRefinement` (so callers attach `.rounded_bl(...)` /
  `.rounded_br(...)` exactly as on any other GPUI element).

Application side wires the right radius per context:

- The main canvas asks for `RADIUS_CARD` (18 px) on its bottom corners
  so it follows the surrounding panel's rounded bottom edge while
  the top stays flush against the toolbar.
- A split pane's canvas asks for the pane's own `SPLIT_PANE_RADIUS`
  (10 px) so the canvas hugs the rounded frame the pane already paints.

`render_failed_web_surface` doesn't need rounding wiring — the error
page is a GPUI div and is clipped normally by the surrounding
`overflow: hidden`.

third_party/gpui/src/platform.rs

@@ -747,7 +747,13 @@ pub(crate) trait PlatformWindow: HasWindowHandle + HasDisplayHandle {
     fn create_native_surface(&self) -> Option<NativeSurfaceHandle> {
         None
     }
-    fn sync_native_surface(&self, _surface: &NativeSurfaceHandle, _bounds: Bounds<Pixels>) {}
+    fn sync_native_surface(
+        &self,
+        _surface: &NativeSurfaceHandle,
+        _bounds: Bounds<Pixels>,
+        _corner_radii: crate::Corners<Pixels>,
+    ) {
+    }
 
     // macOS specific methods
     fn get_title(&self) -> String {