feat: parse YouTube video links and page types

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/lib/video-id.js

@@ -0,0 +1,34 @@
+const ID_RE = /^[A-Za-z0-9_-]{11}$/;
+const YOUTUBE_HOSTS = new Set(['www.youtube.com', 'youtube.com', 'm.youtube.com']);
+
+export function isVideoId(value) {
+  return typeof value === 'string' && ID_RE.test(value);
+}
+
+// Returns { videoId, isShort } for a link to a single video, or null for anything else.
+export function parseVideoLink(href, base = 'https://www.youtube.com') {
+  if (!href) return null;
+  let url;
+  try {
+    url = new URL(href, base);
+  } catch {
+    return null;
+  }
+  if (!YOUTUBE_HOSTS.has(url.hostname)) return null;
+
+  if (url.pathname === '/watch') {
+    const id = url.searchParams.get('v');
+    return isVideoId(id) ? { videoId: id, isShort: false } : null;
+  }
+  const shorts = url.pathname.match(/^\/shorts\/([^/]+)/);
+  if (shorts && isVideoId(shorts[1])) return { videoId: shorts[1], isShort: true };
+  return null;
+}
+
+export function pageFromPath(pathname) {
+  if (pathname === '/') return 'home';
+  if (pathname === '/watch') return 'watch';
+  if (pathname === '/results') return 'search';
+  if (/^\/(@[^/]+|channel\/[^/]+|c\/[^/]+|user\/[^/]+)/.test(pathname)) return 'channel';
+  return null;
+}