Adds progressive web app support for the agents page so it can be
installed as a standalone app on mobile/desktop.
## Changes
- **`manifest.json`** — Web app manifest with `display: standalone`,
`start_url: /agents`, Coder theme colors
- **PWA icons** — 192x192, 512x512 PNGs + 180x180 apple-touch-icon,
rendered from the existing favicon SVG
- **`index.html`** — Added manifest link, apple-touch-icon, and mobile
web app meta tags (`apple-mobile-web-app-capable`,
`mobile-web-app-capable`, `apple-mobile-web-app-status-bar-style`,
title)
- **Service worker** — `notificationclick` now focuses an existing
agents tab or opens `/agents` in a new window
## Testing
1. Open `/agents` on a mobile device
2. Use browser "Add to Home Screen" / "Install App"
3. App should launch in standalone mode pointing at the agents page
4. Push notifications should navigate to the agents page on click
