Guest photos missing for guests without GitHub — add X/Twitter photo fallback

The guests page sources all photos from https://github.com/{guest.github}.png. Guests without a GitHub handle get a broken/default image.

12 guests currently missing photos

Proposed fix

Fall back to https://unavatar.io/twitter/{guest.twitter} when guest.github is null. Loïc Houssier and Anna Poblets would still need a generic placeholder.

Two small changes needed:

• Add twitter to the guest select in src/routes/(site)/guests/+page.server.ts
• Update the <img src> logic in src/routes/(site)/guests/+page.svelte to use the fallback

Start with the guest query in src/routes/(site)/guests/+page.server.ts, then inspect the image source logic in src/routes/(site)/guests/+page.svelte. Add the Twitter field to the selected guest data and use the stated Twitter fallback when GitHub is unavailable. Verify that guests with neither handle receive a generic placeholder and that the listed guests no longer show broken images.