feat: cut clip from Player mode (X/Z keys, scissors button)

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

tests/lib/utils/clipRange.test.ts

@@ -0,0 +1,43 @@
+import { describe, it, expect } from 'vitest';
+import { computeQuickClipRange } from '$lib/utils/clipRange';
+
+describe('computeQuickClipRange', () => {
+  it('returns null when duration is 0', () => {
+    expect(computeQuickClipRange(5, 0, 'at')).toBeNull();
+  });
+
+  it('"at" creates clip from currentTime to currentTime+10', () => {
+    const result = computeQuickClipRange(30, 120, 'at');
+    expect(result).toEqual({ start: 30, end: 40 });
+  });
+
+  it('"before" creates clip from currentTime-10 to currentTime', () => {
+    const result = computeQuickClipRange(30, 120, 'before');
+    expect(result).toEqual({ start: 20, end: 30 });
+  });
+
+  it('"at" clamps end to duration', () => {
+    const result = computeQuickClipRange(115, 120, 'at');
+    expect(result).toEqual({ start: 115, end: 120 });
+  });
+
+  it('"before" clamps start to 0', () => {
+    const result = computeQuickClipRange(3, 120, 'before');
+    expect(result).toEqual({ start: 0, end: 3 });
+  });
+
+  it('accepts custom clip duration', () => {
+    const result = computeQuickClipRange(50, 120, 'at', 20);
+    expect(result).toEqual({ start: 50, end: 70 });
+  });
+
+  it('"at" at time 0 works', () => {
+    const result = computeQuickClipRange(0, 120, 'at');
+    expect(result).toEqual({ start: 0, end: 10 });
+  });
+
+  it('"before" at exact duration works', () => {
+    const result = computeQuickClipRange(120, 120, 'before');
+    expect(result).toEqual({ start: 110, end: 120 });
+  });
+});