Remove dead template/asset files; fix licence.php full-width layout

- Delete templates/header.php and templates/head.php — both were legacy
  partials from a previous design iteration (lang="en", broken nav markup)
  that were never included anywhere in the current codebase.

- Delete public/assets/icons.svg — the full TrumboWYG icon sprite (~15 KB)
  referenced nowhere; the only active WYSIWYG editor (EasyMDE in
  pages-edit.php) loads its own assets from CDN.

- Fix licence.php layout: the page was borrowing the two-column
  .apropos-layout grid but leaving the right column always empty, wasting
  ~40% of the viewport. Removed the grid wrapper and the empty .apropos-right
  div. Added .apropos-single utility class to apropos.css (max-width: 720px)
  so licence content now spans the full available width with a readable
  line length.

public/assets/apropos.css

@@ -29,6 +29,11 @@ html, body {
   max-width: 1200px;
 }
 
+/* Single-column layout — used by licence.php (no right column) */
+.apropos-single {
+  max-width: 720px;
+}
+
 /* Left col — main description text in big monospace */
 .apropos-left {}