Add statistics route, permissions, and sidebar entry

- /statistics route and PagePaths.statistics
- Permission sets: viewer and admin can access /statistics
- Sidebar link with can_access_page check
- Plug and sidebar tests updated

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/mv/authorization/permission_sets.ex

@@ -178,7 +178,9 @@ defmodule Mv.Authorization.PermissionSets do
         # Groups overview
         "/groups",
         # Group detail
-        "/groups/:slug"
+        "/groups/:slug",
+        # Statistics
+        "/statistics"
       ]
     }
   end
@@ -243,7 +245,9 @@ defmodule Mv.Authorization.PermissionSets do
         # Group detail
         "/groups/:slug",
         # Edit group
-        "/groups/:slug/edit"
+        "/groups/:slug/edit",
+        # Statistics
+        "/statistics"
       ]
     }
   end