Fall back to X/Twitter photo for guests without GitHub

Guests with no GitHub handle were showing a broken image.
Falls back to unavatar.io/twitter for guests with a Twitter handle,
and github.com/ghost.png for the rare case of neither.

Closes #2182

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2u841r

src/routes/(site)/guests/+page.server.ts

@@ -16,6 +16,7 @@ export const load = async function () {
 											name_slug: true,
 											id: true,
 											github: true,
+											twitter: true,
 											url: true,
 											social: true
 										}

src/routes/(site)/guests/+page.svelte

@@ -70,7 +70,11 @@
 				<div>
 					<a href="/guest/{guest.name_slug}">
 						<img
-							src="https://github.com/{guest.github || 'null'}.png"
+							src={guest.github
+								? `https://github.com/${guest.github}.png`
+								: guest.twitter
+									? `https://unavatar.io/twitter/${guest.twitter}`
+									: `https://github.com/ghost.png`}
 							alt={guest.name}
 							width="460"
 							height="460"