[DASH-10] PWA Support

[Xflofoxx]

Description

Make dashboard installable as a Progressive Web App.

Requirements

• Installable on desktop/mobile
• Offline support via service worker
• App-like experience
• Push notifications support

Manifest example:

{
  "name": "Pingup Dashboard",
  "short_name": "Pingup",
  "start_url": "/",
  "display": "standalone",
  "background_color": "#111827",
  "theme_color": "#3b82f6"
}

Acceptance Criteria

• PWA installable
• Works offline
• Push notifications work

See: spec/server/DASH-10-pwa-support.md