Specify video/mp4 MIME type on tweet video source

Firefox needs an explicit type on <source> to pick a decoder,
especially when the URL's extension is followed by a query string
(?tag=29) rather than ending cleanly in .mp4. fxtwitter's own
response confirms these are always video/mp4.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014c1L8ghNBFjfiH64UMViP8

frontend/src/lib/components/TweetCard.svelte

@@ -50,7 +50,7 @@
 							muted={item.type === 'gif'}
 							poster={item.thumbnailUrl ? resolveMediaUrl(item.thumbnailUrl) : undefined}
 						>
-							<source src={resolveMediaUrl(item.url)} />
+							<source src={resolveMediaUrl(item.url)} type="video/mp4" />
 						</video>
 					{:else}
 						<img class="media-el" src={resolveMediaUrl(item.url)} alt="" loading="lazy" />