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/models.py

@@ -209,8 +209,14 @@ class LeaveRecord(Base):
     
     Per-employee, per-day, per-type leave. Hours-based granularity (0.5 ~ 8.0).
     HR uploads via CSV/Excel; manually editable.
+    
+    Unique constraint on (employee_name, date, leave_type) ensures one record
+    per employee per day per leave type. Re-import of same record upserts.
     """
     __tablename__ = "leave_records"
+    __table_args__ = (
+        UniqueConstraint("employee_name", "date", "leave_type", name="uix_emp_date_type"),
+    )
 
     id = Column(Integer, primary_key=True, index=True)
     employee_name = Column(String(255), nullable=False, index=True)