Multi-row LeaveModal frontend + LeaveRecord UNIQUE constraint

Frontend (Holidays.jsx LeaveModal):
- Split into Edit Mode (single record) + Create Mode (multi-row)
- Create mode: shared employee/date/approver + list of (type, hours, reason) rows
- Add/remove rows dynamically, each with own type + hours + reason
- Submit loops POST /api/leave per row, aggregates results (added/skipped/failed)
- Submit button label shows count: "加 N 條"
- Bottom shows total valid count + total hours
- Result summary shown after submit (expandable details)
- Auto-close on success after 1.2s

Backend:
- LeaveRecord: add UniqueConstraint (employee_name, date, leave_type)
- Migration: CREATE UNIQUE INDEX uix_leave_emp_date_type (cleaned any pre-existing dups first)
- POST /api/leave: catch IntegrityError → return 409 with clear message\n\nVerified:\n- 3 same-day different-type records (SL/CL/AL) created successfully\n- POST same type twice returns 409 with specific message\n- HTTP 200 for valid creation, 409 for collision

backend/main.py

@@ -1595,7 +1595,13 @@ async def create_leave(
         created_by=current_user.id,
     )
     db.add(lv)
-    db.commit()
+    try:
+        db.commit()
+    except Exception as e:
+        db.rollback()
+        if "UNIQUE" in str(e).upper() or "Duplicate" in str(e):
+            raise HTTPException(status_code=409, detail=f"Leave record exists for {body.employee_name} {body.date} {body.leave_type}")
+        raise
     db.refresh(lv)
     return lv