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

webmention-debug.njk

@@ -48,7 +48,7 @@ pagefindIgnore: true
             {% if allMentions.length > 0 or legacyUrls.length > 0 %}
             <tr class="hover:bg-surface-50 dark:hover:bg-surface-800/50">
               <td class="p-2">
-                <a href="{{ post.url }}" class="text-primary-600 dark:text-primary-400 hover:underline">
+                <a href="{{ post.url }}" class="text-accent-600 dark:text-accent-400 hover:underline">
                   {{ post.url }}
                 </a>
               </td>