Refactor apropos/charte/licence pages: shared layout, TOC anchors, and UI polish

Unify the three public pages (à propos, charte, licence) onto a single
grid layout (.page-content) with sticky TOC sidebar, replacing the old
separate  /  /  markup.

- Merge about.php, charte.php, licence.php templates into shared
  .page-content / .content-section structure
- Add CommonMark HeadingPermalinkExtension for stable heading anchors
- Use SlugNormalizer for TOC links so they match rendered heading IDs
- Standardize link styling across content blocks: bold black, accent on
  hover (consistent with global link style)
- Fix code block wrapping: use pre-wrap instead of pre, constrain grid
  columns with min-width:0, auto scrollbar
- Fix apropos page grid placement: force content-section into column 2
  so contacts and credits stay in the content area, not the sidebar

Also includes accumulated WIP changes:
- Header gradient: hardcoded purple-to-green (replaces CSS variables)
- Search placeholder font
- Duration field: replace minutes/sec/heures with h:m:s time inputs
- TFE file optional for formats 1,4,6 with client-side JS toggle
- Licence form: em-dash to hyphen, details/summary classes
- Pill search: block Enter key form submission when no results
- Draft autosave: remove CSRF rotation (broke concurrent FilePond uploads)
- Language pill: clear hints for excluded main languages
- Search results: gradient placeholder cards for items without covers
- TFE display: format durée values as XhYm instead of decimal

app/src/Controllers/LicenceController.php

@@ -6,6 +6,7 @@ require_once APP_ROOT . '/src/EmailObfuscator.php';
 require_once APP_ROOT . '/src/MarkdownHelper.php';
 
 use League\CommonMark\CommonMarkConverter;
+use League\CommonMark\Extension\HeadingPermalink\HeadingPermalinkExtension;
 
 class LicenceController
 {
@@ -27,7 +28,15 @@ class LicenceController
             $pageTitle = 'Licences';
         }
 
-        $converter = new CommonMarkConverter(['html_input' => 'strip']);
+        $converter = new CommonMarkConverter([
+            'html_input' => 'strip',
+            'heading_permalink' => [
+                'id_prefix' => '',
+                'insert' => 'before',
+                'aria_hidden' => true,
+            ],
+        ]);
+        $converter->getEnvironment()->addExtension(new HeadingPermalinkExtension());
         $html = EmailObfuscator::obfuscateHtml($converter->convert($content)->getContent());
 
         $tocItems = MarkdownHelper::extractToc($content);