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/MarkdownHelper.php

@@ -17,15 +17,14 @@ class MarkdownHelper
     {
         $items = [];
         $lines = explode("\n", $content);
+
+        // Use CommonMark's own SlugNormalizer so TOC links match the rendered heading IDs exactly.
+        $normalizer = new \League\CommonMark\Normalizer\SlugNormalizer();
+
         foreach ($lines as $line) {
             if (preg_match('/^#{1,3}\s+(.+)$/', $line, $m)) {
                 $label = trim($m[1]);
-                // Replicate CommonMark's default heading ID generation:
-                // lowercase, strip non-word chars (except hyphens/spaces), spaces→hyphens
-                $id = strtolower($label);
-                $id = preg_replace('/[^\w\s-]/u', '', $id);
-                $id = preg_replace('/\s+/', '-', $id);
-                $id = trim($id, '-');
+                $id = $normalizer->normalize($label);
                 $items[] = ['label' => $label, 'href' => '#' . $id];
             }
         }