feat(ext): vault-tab Backup & Restore panel

Two cards — Export (passphrase + include-image checkbox → download)
and Restore (file picker + passphrase + new-remote form). Deep-linked
from settings-vault > 'Backup & restore →'.

extension/src/vault/vault.ts

@@ -16,6 +16,7 @@ import { renderDevices, teardown as teardownDevices } from '../popup/components/
 import { renderSettings } from '../popup/components/settings';
 import { renderVaultSettings as renderVaultSettingsView } from '../popup/components/settings-vault';
 import { renderFieldHistory, teardown as teardownFieldHistory } from '../popup/components/field-history';
+import { renderBackupPanel, teardown as teardownBackup } from './components/backup-panel';
 
 // ---------------------------------------------------------------------------
 // Helpers
@@ -66,7 +67,7 @@ function typeLabel(t: ItemType): string {
 // Hash routing
 // ---------------------------------------------------------------------------
 
-type VaultView = 'list' | 'detail' | 'add' | 'edit' | 'trash' | 'devices' | 'settings' | 'settings-vault' | 'field-history';
+type VaultView = 'list' | 'detail' | 'add' | 'edit' | 'trash' | 'devices' | 'settings' | 'settings-vault' | 'field-history' | 'backup';
 
 interface HashRoute {
   view: VaultView;
@@ -92,6 +93,7 @@ function parseHash(): HashRoute {
     case 'settings':
     case 'settings-vault':
     case 'field-history':
+    case 'backup':
       return { view };
     default:
       return { view: 'list' };
@@ -418,6 +420,7 @@ function teardownPaneComponents(): void {
   teardownTrash();
   teardownDevices();
   teardownFieldHistory();
+  teardownBackup();
 }
 
 function renderPane(): void {
@@ -464,6 +467,9 @@ function renderPane(): void {
     case 'field-history':
       renderFieldHistory(pane);
       break;
+    case 'backup':
+      renderBackupPanel(pane);
+      break;
     default:
       pane.className = 'vault-pane vault-pane--empty';
       pane.innerHTML = 'select an item';