feat: replace all primary (blue) with contextual colors across entire theme

Eliminate monotonous blue by replacing ~290 primary- references in 60 files
with semantically appropriate colors:

- accent (teal): links, CTAs, buttons, tabs, focus rings, spinners
- purple: Funkwhale/music, photos, Mastodon/fediverse
- surface (neutral): GitHub, dates/metadata, info boxes
- amber: bookmarks, blogroll categories
- red: likes
- green: reposts
- sky: replies
- orange: RSS/feeds, podcasts
- #0085ff: Bluesky brand
- #a730b8: Mastodon brand

Also updates prose link colors in tailwind.config.js, pagefind UI
primary color to teal, and client-side JS color references.

Confab-Link: http://localhost:8080/sessions/bd3f7012-c703-47e9-bfe2-2ad04ce1842d

404.njk

@@ -8,7 +8,7 @@ pagefindIgnore: true
   <h1 class="text-4xl sm:text-6xl font-bold text-surface-300 dark:text-surface-700 mb-4">404</h1>
   <h2 class="text-xl sm:text-2xl font-semibold text-surface-900 dark:text-surface-100 mb-4">Page Not Found</h2>
   <p class="text-surface-600 dark:text-surface-400 mb-8">Sorry, the page you're looking for doesn't exist. Try searching for it below.</p>
-  <a href="/" class="inline-flex items-center gap-2 px-6 py-3 bg-primary-600 hover:bg-primary-700 text-white font-medium rounded-lg transition-colors mb-8">
+  <a href="/" class="inline-flex items-center gap-2 px-6 py-3 bg-accent-600 hover:bg-accent-700 text-white font-medium rounded-lg transition-colors mb-8">
     <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M10 19l-7-7m0 0l7-7m-7 7h18"/></svg>
     Go back home
   </a>