Build staff management, store details editing, and a forced PIN change

Wires the last two dead buttons in Settings and closes the loop on the
credential work: hashed PINs are only worth having if a shop can actually
change them.

Users & roles (Manage)
- Add, rename, re-role and remove staff. Admin-only at the door, because
  anyone who can edit staff can make themselves an admin.
- PIN and confirmation are both required and must match. There is no email to
  reset a PIN with, so a typo nobody can verify locks the account out until an
  admin intervenes.
- Editing someone leaves their PIN alone unless a new one is typed. An admin
  setting another person's PIN counts as a reset and re-arms must-change.
- Removal is a deactivation with a confirmation that explains why: bills
  already rung keep the cashier's name, so shift reports stay correct.
- Anyone still on a shipped PIN is flagged in the list and in Settings.

Store details (Edit)
- Name, address, GSTIN and phone now editable and persisted. GSTIN is format
  and state-code validated; it prints on every invoice as a legal requirement,
  so a typo is a compliance problem across hundreds of bills.
- Admin-only: changing the GSTIN changes what every future invoice claims
  about who collected the tax.

Forced PIN change
- Shown once after sign-in while must-change is set, and not dismissable. The
  seeded PINs are in the source of the build, so a terminal still running one
  is effectively unprotected.

Fixed while testing: the role dropdown laid its items out at natural width and
"Manager — Sales, inventory and reports" overflowed the dialog by 222px. Now
isExpanded with the description spelled out below, where it is readable.

Tests: 160 -> 168. Covers both role guards, the mismatched and too-short PIN
paths, the default-PIN flag, and GSTIN and seller-name validation.

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

lib/presentation/modules/screens/settings_view.dart

@@ -12,6 +12,8 @@ import '../../../domain/entities/store_account.dart';
 import '../../auth/providers/auth_controller.dart';
 import '../providers/printer_settings.dart';
 import '../widgets/back_office_dialog.dart';
+import '../widgets/staff_dialogs.dart';
+import '../widgets/store_details_dialog.dart';
 import '../../sync/providers/sync_controller.dart';
 import '../widgets/module_widgets.dart';
 
@@ -82,7 +84,10 @@ class _SettingsViewState extends ConsumerState<SettingsView> {
   Widget _storeCard(StoreAccount? store) => PanelCard(
         title: 'Store details',
         subtitle: 'Printed on every invoice',
-        action: TextButton(onPressed: () {}, child: const Text('Edit')),
+        action: TextButton(
+          onPressed: () => showStoreDetailsDialog(context),
+          child: const Text('Edit'),
+        ),
         child: Column(
           mainAxisSize: MainAxisSize.min,
           children: [
@@ -296,7 +301,10 @@ class _SettingsViewState extends ConsumerState<SettingsView> {
 
   Widget _staffCard(StoreAccount? store, StaffUser? current) => PanelCard(
         title: 'Users & roles',
-        action: TextButton(onPressed: () {}, child: const Text('Manage')),
+        action: TextButton(
+          onPressed: () => showStaffDialog(context),
+          child: const Text('Manage'),
+        ),
         child: ResponsiveTable(
           stackBelow: 360,
           columns: const [
@@ -308,10 +316,12 @@ class _SettingsViewState extends ConsumerState<SettingsView> {
               .map((s) => [
                     Cell(s.name, bold: true),
                     Cell(s.role.label, color: AppColors.textSecondary),
-                    s.id == current?.id
-                        ? const TagChip('Signed in',
-                            color: AppColors.success,)
-                        : const SizedBox.shrink(),
+                    if (s.id == current?.id)
+                      const TagChip('Signed in', color: AppColors.success)
+                    else if (s.mustChangePin)
+                      const TagChip('Default PIN', color: AppColors.warning)
+                    else
+                      const SizedBox.shrink(),
                   ],)
               .toList(),
         ),