Split form.css into form-base.css and form-admin.css, drop dead upload-progress code

Also introduces $extraCssAdmin support in head.php for admin-only
stylesheets (form-admin.css, filepond CSS, system.css). Admin pages
now use $extraCssAdmin for admin-only assets and $extraCss for
shared stylesheets like form-base.css.

app/src/Controllers/ThesisEditController.php

@@ -163,7 +163,7 @@ class ThesisEditController
      *                   the transaction is still open, but this method rolls
      *                   back internally before re-throwing).
      */
-    public function save(int $thesisId, array $post, array $files, ?string $progressToken = null): void
+    public function save(int $thesisId, array $post, array $files): void
     {
         if ($thesisId <= 0) {
             throw new InvalidArgumentException('ID de TFE invalide.');