redesign site as "case file" theme

- new paper/ink palette with single red accent, replacing catppuccin
- self-hosted IBM Plex Serif (display) + Source Serif 4 (body), drop Outfit
- rebuilt homepage masthead, numbered post index, navbar with active states
- switch syntax highlighting + utterances to gruvbox, synced via postMessage
- remove terminal/404 easter egg chrome, clean up remaining inline styles
- fix stray syntax.css link causing a 404

templates/partials/comments.html

@@ -1,6 +1,8 @@
 {% if config.extra.enable_comments and config.extra.utterances_repo and page %}
 {% if page.extra.comments is not defined or page.extra.comments %}
-<div class="comments" id="comments" style="margin-top: 4rem;">
+<div class="comments" id="comments"
+    data-utterances-theme-dark="{{ config.extra.utterances_theme | default(value="github-dark") }}"
+    data-utterances-theme-light="{{ config.extra.utterances_theme_light | default(value="github-light") }}">
     <script src="https://utteranc.es/client.js"
             repo="{{ config.extra.utterances_repo }}"
             issue-term="pathname"