Add Accountant role and CanManageBills/CanManageAccounting permissions

- AppConstants: add Accountant to CompanyRoles; add CanManageBills and
  CanManageAccounting to Policies
- ApplicationUser: add CanManageBills and CanManageAccounting bool fields
- UserManagementDtos: expose new fields in all three DTOs
- ClaimsPrincipalFactory: emit ManageBills and ManageAccounting claims
- Program.cs: add CanManageBills and CanManageAccounting policies;
  update CanManageInvoices, CanViewReports, CanManagePurchaseOrders,
  and CanManageVendors to auto-pass for Accountant role
- BillsController: replace CanManageInventory with CanManageBills on
  all write actions (correct policy — bills are not inventory)
- BankReconciliationsController: replace CanManageJobs with
  CanManageAccounting on write actions
- CompanyUsersController: add Accountant to validCompanyRoles (both
  Create/Edit), legacyRole switch, and all permission assignment blocks
- Create/Edit views: add Accountant option to role dropdown; add
  CanManageBills and CanManageAccounting checkboxes; JS auto-checks
  financial permissions when Accountant role is selected
- Migration AddAccountantRolePermissions: adds columns + backfills
  CanManageBills=1 and CanManageAccounting=1 for all CompanyAdmin users

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

src/PowderCoating.Shared/Constants/AppConstants.cs

@@ -31,6 +31,7 @@ public static class AppConstants
     {
         public const string CompanyAdmin = "CompanyAdmin";
         public const string Manager = "Manager";
+        public const string Accountant = "Accountant";
         public const string Worker = "Worker";
         public const string Viewer = "Viewer";
     }
@@ -58,6 +59,8 @@ public static class AppConstants
         public const string CanManageMaintenance = "CanManageMaintenance";
         public const string CanManageInvoices = "CanManageInvoices";
         public const string CanViewReports = "CanViewReports";
+        public const string CanManageBills = "CanManageBills";
+        public const string CanManageAccounting = "CanManageAccounting";
     }
     
     public static class FileUpload