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/vault/vault-status.ts

@@ -5,16 +5,13 @@ import {
   GLYPH_PENDING,
 } from '../shared/glyphs';
 import { relativeTime } from '../shared/relative-time';
+import type { GetVaultStatusResponse } from '../shared/messages';
 
-// Local shape for the sidebar-footer indicator. Mirrors the get_vault_status
-// response data (ahead/behind/lastSyncAt/pendingItems). lastSyncAt is a unix
-// timestamp in SECONDS, or null when the vault has never synced.
-export interface VaultStatus {
-  ahead: number;
-  behind: number;
-  lastSyncAt: number | null;
-  pendingItems: number;
-}
+// The indicator consumes exactly the get_vault_status response payload; alias
+// it (rather than re-declaring the four fields) so the shape stays single-
+// sourced and can't drift from the SW handler. lastSyncAt is a unix timestamp
+// in SECONDS, or null when the vault has never synced.
+export type VaultStatus = GetVaultStatusResponse['data'];
 
 export function renderStatusIndicator(el: HTMLElement, status: VaultStatus): void {
   const ts = status.lastSyncAt !== null