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.Infrastructure/Services/ApplicationUserClaimsPrincipalFactory.cs

@@ -137,6 +137,16 @@ public class ApplicationUserClaimsPrincipalFactory : UserClaimsPrincipalFactory<
             identity.AddClaim(new Claim("Permission", "ViewReports"));
         }
 
+        if (user.CanManageBills)
+        {
+            identity.AddClaim(new Claim("Permission", "ManageBills"));
+        }
+
+        if (user.CanManageAccounting)
+        {
+            identity.AddClaim(new Claim("Permission", "ManageAccounting"));
+        }
+
         return identity;
     }
 }