webapp: fix a11y contrast, heading hierarchy, prefix-safe modal actions

- Accessibility: dark ink (#00003d) on warning/success buttons, badges, and success alert - white text on #ff9500/#0ad64f failed WCAG AA (~2:1); dark-on-bright now passes.
- Heading hierarchy: sidebar brand h1 -> div.brand-title (with matching CSS selector), and standardized all page-header titles to a single <h1> per page (8 templates were h2).
- Prefix-safe modal JS: dashboard/backups/reports/enrollment delete+clone actions now build their form action from url_for(...'__K__'/'__F__').replace(...) instead of hardcoded paths, so they survive a URL-prefix mount.

webapp/templates/audit.html

@@ -3,7 +3,7 @@
 
 {% block content %}
 <div class="page-header">
-  <h2>Audit Log</h2>
+  <h1>Audit Log</h1>
   <div class="header-actions">
     <span class="badge badge-secondary badge-lg">{{ entries|length }} entries</span>
   </div>