fix: use absolute URL in alias redirect to fix 500 error

Response.redirect() requires an absolute URL but was given a relative
path, causing a TypeError and 500 for browser requests to alias routes
like /fullstackjam.

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

Author: fullstackjam

src/hooks.server.ts

@@ -77,7 +77,8 @@ export const handle: Handle = async ({ event, resolve }) => {
 						}
 					}));
 				} else if (isBrowser) {
-					return withSecurityHeaders(Response.redirect(`/${config.username}/${config.slug}`, 302));
+					const baseUrl = env.APP_URL || event.url.origin;
+					return withSecurityHeaders(Response.redirect(`${baseUrl}/${config.username}/${config.slug}`, 302));
 				}
 			}
 		}