Add web app manifest

[jacobgkau]

This allows installing the website as a progressive web app (PWA) on mobile devices and certain web browsers. That's useful to be able to use the app without having the URL bar/tab bar getting in the way.

Relevant docs:

• Mozilla: https://developer.mozilla.org/en-US/docs/Web/Manifest
• Google: https://web.dev/learn/pwa/web-app-manifest

I've tested this on two Android devices (Android 9 and 14).

[Difegue]

Looks good to me. Does it work fine when you start browsing through the web front? (ie edit, reader, etc)
Since we're not a single-page app, I always thought browsers would just start popping additional windows for everything if we declared ourselves as a PWA.

[jacobgkau]

I did test the reader and the admin login before opening the PR to make sure they didn't pop additional windows. On my Android devices, everything stays within the same window.

PWA manifests definitely aren't limited to single-page apps, as Google's docs state:

Caution: If your PWA includes multiple HTML pages, make sure to link its manifest to all of them.

To be on the safe side, I'll go ahead and add the manifest link to the rest of the HTML templates. I'm also changing the spaces to a tab to match the other content in that part of the templates.

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