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/imaging_detail.html

@@ -4,9 +4,9 @@
 {% block content %}
 <div class="page-header">
   <div>
-    <h2>{{ session.serial }}
+    <h1>{{ session.serial }}
       {% if session.hostname_target %}<span class="text-light" style="font-size:14px; font-weight:400;">{{ session.hostname_target }}</span>{% endif %}
-    </h2>
+    </h1>
     <div style="margin-top:0.5rem; display:flex; gap:0.4rem; flex-wrap:wrap;">
       {% if session.pctype %}<span class="badge badge-info">{{ session.pctype }}</span>{% endif %}
       {% if session.machinenumber %}<span class="badge badge-secondary">#{{ session.machinenumber }}</span>{% endif %}