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.

package-lock.json

@@ -23,6 +23,7 @@
         "@radix-ui/react-tooltip": "^1.2.16",
         "@tanstack/react-query": "^5.101.4",
         "@tauri-apps/api": "^2.11.1",
+        "@tauri-apps/plugin-dialog": "^2.7.2",
         "class-variance-authority": "^0.7.1",
         "clsx": "^2.1.1",
         "lucide-react": "^1.27.0",
@@ -2941,6 +2942,15 @@
         "node": ">= 10"
       }
     },
+    "node_modules/@tauri-apps/plugin-dialog": {
+      "version": "2.7.2",
+      "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-dialog/-/plugin-dialog-2.7.2.tgz",
+      "integrity": "sha512-pX0IGm1I3I6wc+zeKYcq1GSqogK6okCNX5fOdaNU5ab1AjGS6l1E5wFNjEb7meg7ZFSp0JUs+0jQGQNyOvLrsg==",
+      "license": "MIT OR Apache-2.0",
+      "dependencies": {
+        "@tauri-apps/api": "^2.11.0"
+      }
+    },
     "node_modules/@tybys/wasm-util": {
       "version": "0.10.3",
       "resolved": "https://registry.npmjs.org/@tybys/wasm-util/-/wasm-util-0.10.3.tgz",