Move Meta tag into root layout

Currently Meta.svelte is imported and rendered manually in individual pages, listing pages, and inside PublicationHead.svelte. Since Meta already self-resolves title/description/tags from PageMeta via the current URL pathname, it can live in the root layout and work for every page automatically.

Current usage

Meta is imported and used in these places:

• +page.svelte -- homepage
• +error.svelte -- error page
• todos/+page.svelte -- todos listing
• features/+page.svelte -- features listing
• PublicationHead.svelte -- all publications
• Several standalone todo pages

Goal

Add <Meta /> to src/routes/+layout.svelte with no props. It will auto-resolve from PageMeta for every page. Then remove all the individual <Meta> usages.

Steps

1. Add Meta to +layout.svelte
2. Remove Meta from PublicationHead
3. Remove Meta from homepage, todos, features
4. Remove Meta from standalone todo pages
5. Handle the error page -- it uses explicit props since error pages don't have PageMeta. Keep Meta there as a fallback, or add error-specific logic to layout Meta.
6. Verify all pages still have correct meta tags

Edge cases

• Error page: passes explicit title/description props. Meta falls back to props when PageMeta is not found, so it could stay in both error page and layout. Check for duplicate title tags.
• Homepage: passes explicit props. Verify PageMeta has the correct title/description.
• Duplicate canonical: root layout already renders canonical link. Meta also renders one when pageMeta.canonical is set. Avoid conflicts.