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/startnet_editor.html

@@ -49,7 +49,7 @@
 
 {% block content %}
 <div class="page-header">
-  <h2>startnet.cmd Editor</h2>
+  <h1>startnet.cmd Editor</h1>
   {% if wim_exists %}
   <div class="header-actions">
     <button type="submit" form="startnetForm" class="pxe-btn pxe-btn-primary">