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/base.html

@@ -31,7 +31,7 @@
   <nav class="sidebar d-flex flex-column">
     <div class="sidebar-header">
       <img class="sidebar-logo" src="{{ url_for('static', filename='ge-aerospace-logo.svg') }}" alt="GE Aerospace">
-      <h1>PXE Manager</h1>
+      <div class="brand-title">PXE Manager</div>
     </div>
 
     <ul class="sidebar-nav nav flex-column mt-2">