refactor: convert admin user management to Server Actions

- Add src/app/actions/admin.ts with updateUserRoles, deleteUser, resetUserPassword
- Update EditUserDialog, DeleteUserDialog, ResetPasswordDialog to use Server Actions
- Remove admin users API routes (PATCH/DELETE/PUT)

docs/server-actions-plan.md

@@ -15,6 +15,9 @@
 | `POST /api/komga/config` | `saveKomgaConfig()` | ✅ Done |
 | `PUT /api/user/password` | `changePassword()` | ✅ Done |
 | `POST /api/auth/register` | `registerUser()` | ✅ Done |
+| `PATCH /api/admin/users/[userId]` | `updateUserRoles()` | ✅ Done |
+| `DELETE /api/admin/users/[userId]` | `deleteUser()` | ✅ Done |
+| `PUT /api/admin/users/[userId]/password` | `resetUserPassword()` | ✅ Done |
 
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