feat: implement role management LiveViews

Add complete CRUD interface for role management under /admin/roles.

- Index page with table showing name, description, permission_set_name, is_system_role
- Show page for role details
- Form component for create/edit with permission_set_name dropdown
- System role badge and disabled delete button
- Flash messages for success/error
- Authorization checks using MvWeb.Authorization helpers
- Comprehensive test coverage (22 tests)

Routes added under /admin scope. All LiveViews load user role
for authorization checks. Form uses custom dropdown for permission sets.

lib/mv_web/router.ex

@@ -81,6 +81,12 @@ defmodule MvWeb.Router do
       live "/contribution_types", ContributionTypeLive.Index, :index
       live "/contributions/member/:id", ContributionPeriodLive.Show, :show
 
+      # Role Management (Admin only)
+      live "/admin/roles", RoleLive.Index, :index
+      live "/admin/roles/new", RoleLive.Form, :new
+      live "/admin/roles/:id", RoleLive.Show, :show
+      live "/admin/roles/:id/edit", RoleLive.Form, :edit
+
       post "/set_locale", LocaleController, :set_locale
     end