- 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)
- Add src/app/actions/library.ts with scanLibrary
- Update ScanButton to use Server Action
- Remove POST from api/komga/libraries/[libraryId]/scan route
- Add src/app/actions/preferences.ts with updatePreferences
- Update PreferencesContext to use Server Action
- Remove PUT from api/preferences route (keep GET)