Android home-screen widgets (Glance) for the same data #70

Part of #67 — Widgets, location and weather

Not API parity — a platform-native expression of the same feature. The web's widgets live on
a dashboard; Android's natural home for them is the launcher.

• Glance-based app widgets for a subset — unread DMs/notifications, next scheduled post, and
  whichever data widgets prove useful.
• Update via the existing WorkManager infrastructure; do not add a second polling scheme.

Scope check: only worth doing once the in-app widget surface exists and the owner confirms the
data matters on mobile.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.