style: rewrite CSS to use Indiekit theme system

Replace all nonexistent CSS variable references with Indiekit's actual
custom properties. This enables automatic dark mode support (variables
swap via prefers-color-scheme) and visual consistency with the rest of
the admin UI.

Key changes:
- Map --color-text → --color-on-background, --color-text-muted →
  --color-on-offset, --border-radius → --border-radius-small, etc.
- Add post-type differentiation via colored left borders: purple for
  notes, green for articles, yellow for boosts, primary for replies
- Replace hardcoded hex colors (#e11d48, #16a34a) with Indiekit's
  palette variables (--color-red45, --color-green50, etc.)
- Use Indiekit's border-width tokens for consistent border sizing
- Add background/color to form inputs for dark mode compatibility

views/partials/ap-item-card.njk

@@ -1,6 +1,6 @@
 {# Timeline item card partial - reusable across timeline and profile views #}
 
-<article class="ap-card">
+<article class="ap-card{% if item.type %} ap-card--{{ item.type }}{% endif %}{% if item.inReplyTo %} ap-card--reply{% endif %}">
   {# Boost header if this is a boosted post #}
   {% if item.type == "boost" and item.boostedBy %}
     <div class="ap-card__boost">