Add stale indicator to documents in the sidebar

Summary Documents that haven't been edited in a long time have no visual signal in the sidebar. Adding a subtle indicator would help users identify content that may need review at a glance. User Story As a team member, I want to see a visual indicator on documents that haven't been updated recently so that I can quickly identify potentially stale content while browsing the sidebar. User Scenario A user opens the sidebar and sees a muted or flagged indicator next to a document that hasn't been edited in over 90 days. They recognize it may be outdated and click in to review it. Acceptance Criteria • Documents not edited within a configurable threshold (default 90 days) show a stale indicator in the sidebar • The indicator is subtle and does not distract from normal browsing • The indicator disappears when the document is edited • The indicator respects the existing isBadgedNew pattern in Document.ts Out of Scope • Configu…