feat: add PBS datastore management and harden macOS keychain storage

Add Proxmox Backup Server datastore management: overview, datastore detail, download/prune/verify/GC dialogs, usePbs hook, backend commands, and tests.

Fix macOS keychain re-writes failing with 'item already exists': replace keyring 3 with keyring-core plus per-platform stores (macOS Keychain, Windows Credential Manager, Linux keyutils), recover by deleting the stale item and retrying once, and surface actionable messages for locked keychains.

src-tauri/tests/api_commands.rs

@@ -39,6 +39,7 @@ async fn setup_manager(
         username: None,
         nodes: vec![],
         cluster_id: None,
+        server_type: "pve".to_string(),
     };
     manager
         .add_connection(config, &path)