panel: move inline styles and confirmations out of the templates

Groundwork for the Content-Security-Policy of phase 14.A. A policy that has to
allow inline script is not worth writing — script-src 'unsafe-inline' gives
back exactly the XSS foothold the policy exists to remove — so the three
inline constructs the templates still had are moved out first:

  - the layout's <style> block becomes /static/panel.css;
  - the one style="background:#b42318" attribute becomes the .danger class
    that already existed for it;
  - the four onsubmit="return confirm(...)" handlers become data-confirm,
    handled by a delegated listener in panel.js. Delegation matters: the
    application rows are also delivered by HTMX swaps.

htmx would otherwise inject a <style> of its own for the request-indicator
classes and become the single reason the policy needs an exemption; the panel
uses no hx-indicator, so the meta config switches it off.

A guard test keeps this from silently regressing later, which it otherwise
would: an inline handler added to a template does not fail, it just quietly
stops working in the browser.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/web/templates/domain_delete.html

@@ -18,7 +18,7 @@
   invalid; remove it from DNS if you do not plan to re-add the domain.</p>
 
   <form method="post" action="/domains/{{.Domain.ID}}/delete">
-    <button type="submit" style="background:#b42318">Delete {{.Domain.Name}}</button>
+    <button type="submit" class="danger">Delete {{.Domain.Name}}</button>
   </form>
 </div>
 {{end}}