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/licence.php

@@ -47,17 +47,12 @@ $html = $pd->text($content);
     <?php include APP_ROOT . '/templates/search-bar.php'; ?>
 
     <main class="apropos-main" id="main-content">
-        <div class="apropos-layout">
-            <div class="apropos-left">
-                <div class="apropos-description apropos-page-content">
-                    <?php if (!empty(trim($content))): ?>
-                        <?= $html ?>
-                    <?php else: ?>
-                        <p>Contenu à venir.</p>
-                    <?php endif; ?>
-                </div>
-            </div>
-            <div class="apropos-right"></div>
+        <div class="apropos-description apropos-page-content apropos-single">
+            <?php if (!empty(trim($content))): ?>
+                <?= $html ?>
+            <?php else: ?>
+                <p>Contenu à venir.</p>
+            <?php endif; ?>
         </div>
     </main>