feat: add basic pwa

[userquin]

DON'T MERGE YET (I cannot create draft PR)

This PR includes offline support, added fallback page and prerender it, we can change the name later.

The PWA is registered in dev an on locla build, not yet on deployment:

• the sw in dev will only intercept the fallback
• on offline, Elk will work but the sw will redirect to fallback (something similar to twitter/mastodon: works with the SPA but if you refresh the dinossaur will appear)
• the current strategy is prompt for update: we can change it later, I need to review the dialog/toast
• if we want to cache also avatars for users we'll need to add cors and change all img tag avatars adding crossorigin="anonymous"; this is not about sponsor avatars (antfu, danielroe, patak...) it is about matodon users

closes #101

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[userquin]

superseded by #337