css: replace presentational class selectors with semantic element selectors

Replace 6 CSS class selectors across tfe.css, main.css, and search.css with
semantic element-based selectors, removing the corresponding classes from the
HTML templates entirely.

tfe.css:
- .tfe-meta-list → article dl / article dl > div / article dl dt / article dl dd
- .tfe-media-block → aside figure (+ img, video, embed children)
- .tfe-file-caption → aside figcaption

main.css:
- .card__media → .home-body figure (+ img/video children and hover/motion rules)
- .card__caption → .home-body li > a > p

search.css:
- .repertoire-col > h2 → .repertoire-index section > h2

Template changes:
- tfe.php: removed class= from <dl>, <figure>, and <figcaption>
- index.php: removed class= from <figure> and <p class=card__caption>;
  stripped orphaned card__media from the gradient <div> (only --gradient needed)

No visual change — selectors match the same elements as before since the
semantic HTML was already in place from prior refactoring work.

public/index.php

@@ -99,7 +99,7 @@ $bodyClass = 'home-body';
                         // 3. Fall through to gradient
                         ?>
                         <?php if ($thumb): ?>
-                            <figure class="card__media">
+                            <figure>
                                 <img src="/media.php?path=<?= urlencode($thumb) ?>"
                                      alt="Couverture — <?= htmlspecialchars($item['title']) ?> par <?= htmlspecialchars($item['authors'] ?? '') ?>"
                                      loading="lazy">
@@ -109,14 +109,14 @@ $bodyClass = 'home-body';
                             $hue  = ($item['id'] * 47 + 160) % 360;
                             $hue2 = ($hue + 40) % 360;
                             ?>
-                            <div class="card__media card__media--gradient"
+                            <div class="card__media--gradient"
                                  style="background:linear-gradient(135deg,hsl(<?= $hue ?>,55%,40%),hsl(<?= $hue2 ?>,50%,28%));"
                                  aria-hidden="true">
                                 <span class="card__gradient-author"><?= htmlspecialchars($item['authors'] ?? '') ?></span>
                                 <span class="card__gradient-title"><?= htmlspecialchars($item['title']) ?></span>
                             </div>
                         <?php endif; ?>
-                        <p class="card__caption"><?= htmlspecialchars($item["authors"] ?? '') ?><?php if (!empty($item['authors']) && !empty($item['title'])): ?> – <?php endif; ?><?= htmlspecialchars($item["title"]) ?></p>
+                        <p><?= htmlspecialchars($item["authors"] ?? '') ?><?php if (!empty($item['authors']) && !empty($item['title'])): ?> – <?php endif; ?><?= htmlspecialchars($item["title"]) ?></p>
                     </a>
                 </li>
             <?php endforeach; ?>