fix(annotations): make resize overlay keyboard-interactive

- Add tabindex="0" so the SVG can receive DOM focus
- Auto-focus the SVG on mount so arrow keys work immediately after
  clicking an annotation to select it
- Show preview rect during keyboard nudging (not just pointer drag) by
  checking hasLiveChanges instead of only checking dragState
- Suppress default browser focus outline (outline: none) on the SVG

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/AnnotationEditOverlay.svelte.test.ts

@@ -60,4 +60,12 @@ describe('AnnotationEditOverlay', () => {
 		const liveRegion = document.querySelector('[aria-live="polite"]');
 		expect(liveRegion).not.toBeNull();
 	});
+
+	it('SVG root has tabindex="0" so it can receive keyboard focus', async () => {
+		render(AnnotationEditOverlay, { annotation });
+
+		const svg = document.querySelector('svg[role="application"]');
+		expect(svg).not.toBeNull();
+		expect(svg!.getAttribute('tabindex')).toBe('0');
+	});
 });