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/src/error.rs

@@ -41,6 +41,9 @@ pub enum Error {
 
     #[error("Tauri error: {0}")]
     TauriError(#[from] tauri::Error),
+
+    #[error("I/O error: {0}")]
+    IoError(#[from] std::io::Error),
 }
 
 impl Serialize for Error {