refactor(ext): simplify Phase 6 — alias VaultStatus + reuse listItems

Two simplify-pass cleanups:
- vault-status.ts: VaultStatus is now an alias of GetVaultStatusResponse['data']
  instead of a re-declared 4-field interface, so the renderer's input shape is
  single-sourced from the message contract and can't drift from the SW handler.
- service-worker/vault.ts: handleGetVaultStatus counts active items via the
  existing listItems() helper rather than re-implementing the trashed_at filter.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/src/service-worker/vault.ts

@@ -566,9 +566,7 @@ export function handleGetVaultStatus(
   },
 ): GetVaultStatusResponse | { ok: false; error: string } {
   if (!state.gitHost) return { ok: false, error: 'vault_locked' };
-  const pendingItems = state.manifest
-    ? Object.values(state.manifest.items).filter((e) => e.trashed_at === undefined).length
-    : 0;
+  const pendingItems = state.manifest ? listItems(state.manifest).length : 0;
   return {
     ok: true,
     data: {