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/capabilities/default.json

@@ -14,6 +14,7 @@
     "core:window:allow-hide",
     "core:window:allow-close",
     "core:window:allow-set-focus",
-    "core:window:allow-is-visible"
+    "core:window:allow-is-visible",
+    "dialog:default"
   ]
 }