feat: add UI-level authorization helpers

Implement MvWeb.Authorization module with can?/3 and can_access_page?/2
functions for conditional rendering in LiveView templates.

- can?/3 supports both resource atoms and record structs with scope checking
- can_access_page?/2 checks page access permissions
- All functions use PermissionSets module for consistency with backend
- Graceful handling of nil users and invalid permission sets
- Comprehensive test coverage with 17 test cases

lib/mv_web.ex

@@ -89,6 +89,9 @@ defmodule MvWeb do
       # Core UI components
       import MvWeb.CoreComponents
 
+      # Authorization helpers
+      import MvWeb.Authorization, only: [can?: 3, can_access_page?: 2]
+
       # Common modules used in templates
       alias Phoenix.LiveView.JS
       alias MvWeb.Layouts