style: differentiate bluesky/x source badges in feed with theme-aware colors

The .source-bluesky and .source-x classes were applied dynamically but had
no corresponding CSS rules, making both sources look identical. Now Bluesky
posts get the theme accent color and X posts get a muted tone, both via
CSS custom properties so all three themes are covered.

https://claude.ai/code/session_01SDyH2mc5QWKqDwoZDW1ehW

Author: claude

pages/feed.vue

@@ -281,6 +281,17 @@ h1 {
   border-radius: 4px;
   padding: 1px 5px;
   white-space: nowrap;
+  transition: color 0.15s ease, border-color 0.15s ease;
+}
+
+.source-bluesky {
+  color: var(--theme-accent, #6b8cae);
+  border-color: color-mix(in srgb, var(--theme-accent, #6b8cae) 35%, transparent);
+}
+
+.source-x {
+  color: var(--theme-text-muted, #888);
+  border-color: color-mix(in srgb, var(--theme-text-muted, #888) 35%, transparent);
 }
 
 .post-stats {