Add stale indicator to documents in the sidebar

[cpaylward]

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 They recognize it may be outdated and click in to review it.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

• Configurable threshold per workspace (separate issue)
• Notifications or emails about stale documents
• Stale indicators on collections

Open Questions

• What should the default be? 90 days? 180 days?
• Should the indicator appear on nested documents in collapsed sidebar sections?