feat: implement native OS keyring for site logins

- Replaced plaintext local storage password fields with keyring v3 OS native integration.
- Tauri backend securely sets, gets, and deletes credentials via system keychains.
- Dynamic password injection occurs seamlessly on download start and metadata parsing.

apps/desktop/src/store/useSettingsStore.ts

@@ -5,7 +5,6 @@ export interface SiteLogin {
   id: string;
   urlPattern: string;
   username: string;
-  password?: string;
 }
 
 export type AppFontSize = 'small' | 'standard' | 'large';