feat: add file display to forms and recap pages

- Live file preview on all file inputs (file-field partial, edit template):
  thumbnails for images, emoji icons for PDF/video/zip/vtt, filename + size
- New file-preview.js wired via $extraJs in add.php / edit.php and direct
  <script> in partage/index.php; $extraJs support added to head.php
- admin/recapitulatif.php: replace plain table with rich file list — image
  thumbnails linked to media.php, type badges, human-readable size, date
- partage/recapitulatif.php: full rewrite — shows thesis metadata + files
  list with same rich display (no media links for student privacy)
- form.css: new sections for .file-preview-list (live preview) and
  .recap-file-list / .recap-dl / .partage-recap (recap pages)

app/templates/head.php

@@ -69,6 +69,9 @@
     <?php foreach ($extraCss ?? [] as $css): ?>
     <link rel="stylesheet" href="<?= App::assetV($css) ?>">
     <?php endforeach; ?>
+    <?php foreach ($extraJs ?? [] as $js): ?>
+    <script src="<?= App::assetV($js) ?>" defer></script>
+    <?php endforeach; ?>
     <?php if (php_sapi_name() === 'cli-server'): ?>
     <script>
         (function poll(){