feat: lead-in margin, 0-indexed bars, fix bar number clipping

- Add 1-bar lead-in margin at timeline start (bar 0, time 0s at margin edge)
- Playhead position includes lead-in offset
- Bar numbers now 0-indexed (0, 1, 2...) in ruler and track grid
- Fix bar number clipping: CLIP_BUFFER increased from 200px to
  max(400px, barWidth+200px) so bars past viewport edge render
- All mouse/time handlers account for lead-in margin (ruler, clip
  drag, section drag/resize, context menu, double-click)
- formatBeat returns 0-indexed bar
- Sub-tab ruler has leadInMargin=0 (no lead-in in clip editor)

wiki.md

@@ -79,3 +79,9 @@
 - **Các file ảnh hưởng:** `app/static/js/app.jsx`
 - **Ghi chú/Test (nếu có):** `npm run build` — build passes.
 ---
+
+### [2026-07-25 10:18] Task: Lead-in margin, bar 0-indexed, grid clip fix
+- **Tóm tắt thay đổi:** Thêm leadInMargin (1 bar) đẩy timeline content sang phải — bar 0, time 0s tại mép margin. Playhead bắt đầu từ đó. Bar numbering 0-indexed (bar 0 thay vì 1). Sửa CLIP_BUFFER từ 200px → max(400, barWidth+200) để bar number không bị che. Đồng bộ leadInMargin vào tStart/tEnd canvas + mouse handlers (ruler, clip drag, section resize).
+- **Các file ảnh hưởng:** `app/static/js/app.jsx`
+- **Ghi chú/Test (nếu có):** `npm run build` — build passes.
+---