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/public/admin/add.php

@@ -46,6 +46,7 @@ function wasSelected($key, $value) {
 $isAdmin = true;
 $bodyClass = 'admin-body';
 $extraCss  = ['/assets/css/form.css'];
+$extraJs   = ['/assets/js/file-preview.js'];
 require_once APP_ROOT . '/templates/head.php';
 include APP_ROOT . '/templates/header.php';
 include APP_ROOT . '/templates/admin/add.php';