Fix billing data integrity, sale atomicity and stock safety

Bills were persisted correctly but read back wrong. The read path rebuilt a
cart from its lines alone, dropping bill-level discounts and loyalty, so every
figure derived from a stored bill was overstated: the upload payload, the day
archive and the shift report. A discounted 529 bill read back as 620.

Money and data integrity
- order_dao: restore bill_discount and points_redeemed when rebuilding a cart;
  keep the reconstruction tier-less so the membership discount is not applied
  twice. Trust the recorded total and points via SaleTransaction.storedTotal.
- checkout_sale + order_dao.commitSale: write the bill, its stock movement and
  the loyalty update in one transaction. Previously a failure part-way through
  left a persisted bill the cashier believed had failed, inviting a duplicate.
- checkout_sale: re-check every line against live stock. A parked bill resumed
  after its stock was sold passed validation and oversold.
- catalogue_dao: allocate the invoice sequence in one transaction; the previous
  read-modify-write could hand two sales the same number and fail UNIQUE.
- local_store: replay unsynced sales after a catalogue import, so a mid-shift
  re-import cannot restore stock that has already been sold.
- payment_controller: stamp the signed-in operator on the bill instead of the
  hardcoded seed session, and pass the terminal id through.
- cart: reconcile per-slab GST against the bill total so the parts sum to the
  whole on a tax invoice.

Sync and reporting
- sync_repository: drain unsynced bills in a loop rather than silently capping
  at one page; stop on rejection so rejected rows cannot loop forever.
- sync_log_dao (new): persist the sync history to the sync_log table, which the
  schema already defined but nothing used. It was in memory, so the only record
  that bills had been uploaded died at restart.
- Scope shift reports by cashier. day_archive is re-keyed to
  (business_date, cashier_name) so a till stays settleable after its bills are
  uploaded and deleted. Schema v4 with a migration that carries v3 rows across.

Input and UI
- barcode_service: consume machine-paced keystrokes so a scan cannot also land
  in the focused field, and raise the bar to 60ms/char while a text field has
  focus so typing a mobile number is not read as a scan. Clock and focus check
  injected so the behaviour is testable.
- primary_button: make the label flexible; label plus trailing total overflowed
  the Charge button by up to 131px.
- app_router: redirect instead of null-casting when the receipt route is
  entered without its transaction.
- customer_repository: reduce the search query to digits so a punctuated mobile
  number matches.

Cleanup
- Remove TransactionRepository.save, CustomerRepository.recordSale and
  OrderDao.insertOrder, all superseded by commitSale.
- dart fix across the tree; 251 analyzer issues down to 3 info-level.

Tests: 23 passing / 15 failing -> 90 passing. Fixed the two defects that broke
the existing suite (containsAll type argument, reset() needing a catalogue) and
deleted the leftover template test. Added coverage for the order round trip,
the day archive after a real sync, stock safety, checkout atomicity, the v3->v4
migration, scanner-versus-human input, and an app-level smoke test that renders
every module.

Note: bills already uploaded with a discount went up overstated. This stops it
happening again but does not correct historical server data.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/domain/entities/cart.dart

@@ -174,11 +174,29 @@ class Cart extends Equatable {
   double get taxableAmount => (netAmount - taxAmount).asMoney;
 
   /// GST broken out per slab — required on a compliant tax invoice.
+  ///
+  /// The slabs are reconciled against [taxAmount] before being returned.
+  /// Rounding each slab on its own leaves the parts summing to a paisa either
+  /// side of the total printed on the same bill, which a tax invoice cannot
+  /// show; the residue is absorbed by the largest slab.
   Map<double, double> get taxBreakdown {
-    final map = <double, double>{};
+    final raw = <double, double>{};
     for (final line in lines) {
       final rate = line.product.gstRate;
-      map[rate] = ((map[rate] ?? 0) + line.taxAmount * _billFactor).asMoney;
+      raw[rate] = (raw[rate] ?? 0) + line.taxAmount * _billFactor;
+    }
+    if (raw.isEmpty) return const {};
+
+    final map = {
+      for (final e in raw.entries) e.key: e.value.asMoney,
+    };
+
+    final drift =
+        (taxAmount - map.values.fold(0.0, (a, b) => a + b)).asMoney;
+    if (drift != 0) {
+      final largest =
+          raw.entries.reduce((a, b) => a.value >= b.value ? a : b).key;
+      map[largest] = (map[largest]! + drift).asMoney;
     }
     return map;
   }