Add feed link tags to layout

The feed link tags in +layout.svelte were commented out during initial setup and never restored. Without <link rel="alternate"> tags, feed readers and browsers cannot auto-discover the site's JSON and XML feeds.

Current State

• feed.json and feed.xml routes are restored and working
• +layout.svelte has no <link rel="alternate"> tags pointing to either feed

Tasks

• Add <link rel="alternate" type="application/feed+json" href="/feed.json"> to <svelte:head> in +layout.svelte
• Add <link rel="alternate" type="application/atom+xml" href="/feed.xml"> to <svelte:head> in +layout.svelte