Fix dark mode color contrast on server cert settings page

UX Fix: Dark mode readability

Problem:
- Warning banner: light yellow background with dark text unreadable in dark mode
- Code blocks: light gray background with dark text unreadable in dark mode
- User reported: 'warning on darkmode not visible, light yellow background on white text'

Solution:
- Added @media (prefers-color-scheme: dark) styles
- Warning banner: dark background (#664d03), light text (#ffecb5)
- Code blocks: dark background (#2b2b2b), light text (#e0e0e0)
- Maintains good contrast in both light and dark themes

Colors:
- Light mode warning: #fff3cd background, #856404 text (WCAG AA compliant)
- Dark mode warning: #664d03 background, #ffecb5 text (high contrast)
- Light mode code: #f8f9fa background, #212529 text
- Dark mode code: #2b2b2b background, #e0e0e0 text

Deployment Note:
After editing plugins/*/frontend/ files, run:
  node scripts/stage-frontend.mjs

This copies plugin frontends to frontend/src/.plugins-staged/
where Vite reads them (per ADR-013 plugin pruning).

Context:
- Missed running staging script initially (my error)
- Manual copy to .plugins-staged worked but wrong workflow
- .plugins-staged/ is .gitignored (generated, not committed)
- Proper workflow: edit source -> stage -> Vite hot-reloads

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

plugins/endpoints/frontend/views/ServerCertSettings.vue

@@ -419,6 +419,16 @@ function formatDate(dateString) {
   border-radius: 4px;
   padding: 12px;
   margin-bottom: 16px;
+  color: #856404;
+}
+
+/* Dark mode support */
+@media (prefers-color-scheme: dark) {
+  .warning-banner {
+    background: #664d03;
+    border-color: #ffca2c;
+    color: #ffecb5;
+  }
 }
 
 .expiry-warning {