Add optional Alternative.image and Feature.color/icon for richer layouts

image: a banner url rendered as <img>, client-fetched directly (never
server-side, so none of ResourceSource::Url's SSRF concern applies).
color: any CSS color, set as the feature's own --feature-accent custom
property (never interpolated into a stylesheet) - draws a left-border
accent; unset means no border, not a fallback to the global --accent.
icon: an Iconify name (lucide:star etc.), rendered via Iconify's public
SVG API - no icon library bundled, matching this app's zero-JS-
dependency content otherwise keeps to.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

style/main.css

@@ -280,6 +280,15 @@ main.not-found {
   padding: 1.75rem 1.9rem;
 }
 
+.alt-image {
+  display: block;
+  width: 100%;
+  max-height: 14rem;
+  object-fit: cover;
+  border-radius: calc(var(--radius) - 0.3rem);
+  margin-bottom: 1.1rem;
+}
+
 .alt-card h2 {
   font-size: 1.3rem;
 }
@@ -316,6 +325,18 @@ main.not-found {
   margin-top: 0.6rem;
 }
 
+.feature {
+  border-left: 3px solid var(--feature-accent, transparent);
+  padding-left: 0.9rem;
+}
+
+.feature-icon {
+  float: left;
+  width: 1.3rem;
+  height: 1.3rem;
+  margin: 0.1rem 0.6rem 0.4rem 0;
+}
+
 .feature h3 {
   font-size: 1rem;
   margin-bottom: 0.15em;