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/templates/public/search.php

@@ -76,15 +76,22 @@
     <ul class="results-grid">
         <?php foreach ($results as $item): ?>
         <?php $thumb = $coverMap[$item['id']] ?? null; ?>
-        <li><a href="/tfe?id=<?= (int)$item['id'] ?>" class="result-card<?= $thumb ? ' result-card--has-cover' : '' ?>">
+        <li><a href="/tfe?id=<?= (int)$item['id'] ?>" class="result-card">
             <?php if ($thumb): ?>
             <figure class="result-card__cover">
                 <img src="/media?path=<?= urlencode($thumb) ?>"
                      alt="Couverture — <?= htmlspecialchars($item['title']) ?>"
                      loading="lazy">
             </figure>
+            <?php else: ?>
+            <div class="result-card__gradient">
+                <span class="result-card__gradient-author"><?= htmlspecialchars($item['authors'] ?? '') ?></span>
+                <span class="result-card__gradient-title"><?= htmlspecialchars($item['title']) ?></span>
+            </div>
             <?php endif; ?>
+            <?php if ($thumb): ?>
             <span class="result-card__authors"><?= htmlspecialchars($item['authors'] ?? '') ?></span>
+            <?php endif; ?>
             <span class="result-card__title"><?= htmlspecialchars($item['title']) ?></span>
             <small class="result-card__meta"><?= htmlspecialchars($item['year']) ?><?php if (!empty($item['orientation'])): ?> · <?= htmlspecialchars($item['orientation']) ?><?php endif; ?></small>
         </a></li>