fix(path-picker): finish the image preview zoom from #608

The review items offered on #608, applied on top of the contributor's
zoom:

- `object-fit: scale-down` instead of `contain`, so a small image keeps
  its own size instead of being stretched to fill the preview. The pan
  clamp now uses the size the image is actually drawn at (natural size
  fitted with scale-down) rather than the image box, which let a small
  or letterboxed image be panned entirely out of view.
- Wheel zoom is proportional to deltaY (d3-zoom's scale: about 15% per
  100px notch, lines and ctrl+wheel trackpad pinches scaled up), capped
  at sqrt(2) per event, and a wheel event with no vertical delta zooms
  nothing. A trackpad scroll no longer jumps to 600%.
- The zoom controls show only once the image has loaded; on an error
  they are hidden and the gesture listeners are dropped with the image.
  Both handlers now check the request sequence like the text branch, so
  a late load or error from a replaced preview no longer resets the
  current preview's zoom or paints its error over the current image.
- test/path-picker-image-zoom.test.ts drives the real listeners in
  jsdom (focal point under the cursor and the pinch centre, pan clamp,
  wheel scaling, error, close and stale events) in place of the
  source-string checks in test/path-picker-ui.test.ts. 7 of its 10
  tests fail against the contributor's commit alone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/path-picker-ui.test.ts

@@ -169,15 +169,6 @@ describe('mobile filesystem picker actions', () => {
     expect(keyboardSource).toContain('pre.textContent = content');
   });
 
-  it('supports pinch zoom, pan, and explicit zoom controls in image previews', () => {
-    expect(keyboardSource).toContain("entry.previewKind === 'image'");
-    expect(keyboardSource).toContain("listen(image, 'pointerdown'");
-    expect(keyboardSource).toContain("listen(image, 'pointermove'");
-    expect(keyboardSource).toContain("listen(image, 'dblclick'");
-    expect(keyboardSource).toContain('data-zoom="fit"');
-    expect(keyboardSource).toContain('this._previewImageCleanup?.()');
-  });
-
   it('inserts a selected path into the editable local-echo prompt without sending it', () => {
     const appendText = vi.fn();
     const sendInput = vi.fn();