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/cluster_flow.rs

@@ -124,6 +124,7 @@ fn token_config(id: &str, url: &str, fallbacks: Vec<EndpointConfig>) -> Connecti
         username: None,
         nodes: vec![],
         cluster_id: None,
+        server_type: "pve".to_string(),
     }
 }