fix: guard against zero-length clip at EOF

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/lib/utils/clipRange.test.ts

@@ -40,4 +40,8 @@ describe('computeQuickClipRange', () => {
     const result = computeQuickClipRange(120, 120, 'before');
     expect(result).toEqual({ start: 110, end: 120 });
   });
+
+  it('"at" returns null when currentTime equals duration', () => {
+    expect(computeQuickClipRange(120, 120, 'at')).toBeNull();
+  });
 });