site: founding post, headline post layout, masonry grid, source links

- New "Why this exists" post; masthead "founded" links to it
- Post template: serif headline + date in masthead, "Filed under {category}" in footer
- Drop counts from footer Collections list
- Masonry component for home + sources grid; source badges link to /sources/#slug
- Theme toggle (settings page + cog) with dark-mode-aware tokens
- Six new themed find lists; cluster-finds skill

src/lib/sources.ts

@@ -0,0 +1,6 @@
+export function sourceSlug(name: string): string {
+  return name
+    .toLowerCase()
+    .replace(/[^a-z0-9]+/g, '-')
+    .replace(/^-+|-+$/g, '');
+}