feat: Phase 2E — cash drawer reconciliation

Backend:
- WaiterShift: add counted_cash_end (physical cash handed over) + cash_discrepancy (computed)
- BusinessDay: add store_opening_cash, store_closing_cash, store_cash_discrepancy
- EndShiftRequest: accept counted_cash_end (optional)
- CloseBusinessDayRequest: accept store_closing_cash (optional)
- Shift end (both /end and /manager/end/{id}): compute cash_discrepancy =
  counted_cash_end - (starting_cash + total_collected); negative = short, positive = over
- Business day close: compute store_cash_discrepancy = store_closing_cash - expected;
  expected = store_opening_cash + sum(shift.total_collected for day)
- _enrich_shift: expose counted_cash_end + cash_discrepancy in all shift responses
- Business day summary: expose store cash fields + store_expected_closing + total_waiter_collected
- BusinessDayOut schema: expose new store cash fields
- Shifts CSV export: add counted_cash_end + cash_discrepancy columns

Frontend:
- EndShiftConfirmModal: 2-step flow — step 1 notes, step 2 cash handover with live discrepancy
  (shows ✓ clean / ⚠ short / ⚠ over in real time before confirming)
- WorkdaySummaryModal close flow: new Ταμείο tab with store cash count + live discrepancy;
  footer navigates overview → Ταμείο → Έλεγχοι; store_closing_cash included in close payload
- ShiftsOverview report: Ταμείο column showing counted_cash_end + discrepancy badge
- ShiftDetailModal row 2: replaces Εξ.Τραπεζιών with Παραδόθηκαν + Ταμείο discrepancy

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local_backend/main.py

@@ -273,6 +273,12 @@ def _run_migrations():
         # Phase 2B — staff payroll
         "ALTER TABLE users ADD COLUMN hourly_rate REAL",
         "ALTER TABLE waiter_shifts ADD COLUMN hourly_rate_snapshot REAL",
+        # Phase 2E — cash drawer reconciliation
+        "ALTER TABLE waiter_shifts ADD COLUMN counted_cash_end REAL",
+        "ALTER TABLE waiter_shifts ADD COLUMN cash_discrepancy REAL",
+        "ALTER TABLE business_days ADD COLUMN store_opening_cash REAL",
+        "ALTER TABLE business_days ADD COLUMN store_closing_cash REAL",
+        "ALTER TABLE business_days ADD COLUMN store_cash_discrepancy REAL",
         # Phase 2D — expense tracking + supplier contacts
         """CREATE TABLE IF NOT EXISTS contacts (
             id INTEGER PRIMARY KEY AUTOINCREMENT,