feat(frontend): add pagination and filters to admin users table

- Remove deprecated auth type tabs (All/Email/GitHub)
- Add server-side pagination with 20 items per page default
- Add search type selector (Username/Email) using ButtonGroup pattern
- Add DropdownMenu for filters matching MCP catalog pattern
- Add auth_type filter with email_signup and github options
- Add role filter with global_admin and global_user options
- Implement active filters row with inline pill components
- Add loading states (skeleton + spinners)
- Update UserService with getUsersPaginated() and searchUsers() methods
- Update types for paginated API responses
- Remove search button, search triggers on Enter key only
- Update pagination alignment to match MCP catalog (right-aligned)
- Fix auth_type filter value to use 'email_signup' instead of 'email'

Author: Lasim

services/backend/api-spec.json

@@ -2124,6 +2124,7 @@
               "type": "string",
               "enum": [
                 "email",
+                "email_signup",
                 "github"
               ]
             },

services/backend/api-spec.yaml

@@ -1494,6 +1494,7 @@ paths:
             type: string
             enum:
               - email
+              - email_signup
               - github
           in: query
           name: auth_type

services/backend/src/routes/users/schemas.ts

@@ -277,7 +277,7 @@ export const SEARCH_USERS_QUERY_SCHEMA = {
     },
     auth_type: {
       type: 'string',
-      enum: ['email', 'github'],
+      enum: ['email', 'email_signup', 'github'],
       description: 'Filter by authentication type'
     },
     role_id: {