Add web app manifest and offline page - #2009

Replaces #1960.

• `manifest.json` with relative URLs, so subfolder installs work.
• Icons from Add Progressive Web App (PWA) support #1960, plus a padded maskable icon.
• `theme-color`, `apple-mobile-web-app-capable` and manifest link in `Theme/theme.php`.
• `service-worker.js` handles page loads only. Shows `Theme/offline.html` when the network fails. API, POST and asset requests are not intercepted. Only the offline page is cached.

Differences from #1960:

• No caching of pages or API responses, so live data, logout and user separation are unaffected.
• Asset caching left to the existing `?v=filemtime` versioning.

Notes:

• Service workers need HTTPS or localhost. On plain http (typical emonPi LAN access) the worker does not register and install falls back to a shortcut.
• On iOS an installed app has its own cookies, so users log in once inside the app.

Tested on localhost with Chrome: manifest and icons load, worker registers with scope at the emoncms root, offline page shown when offline, normal login page when back online.

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