Refactor HTMX fragment architecture: DRY split into auth endpoints + shared templates

- Created templates/partials/form/_licence.php (shared HTML, no auth logic)
- Created templates/partials/form/_format-website.php (shared HTML, no auth logic)
- Created src/FragmentRenderer.php helper for clean fragment rendering
- Created public/{admin,partage}/fragments/ subdirectories
- Created thin fragment endpoint files: auth guard + data fetch + render template
- Updated all hx-post references in templates to new fragments/ paths
- Updated partage/index.php routing for new fragments subdirectory
- Kept old fragment files as thin delegates for backward compat
- Updated nginx config: added PHP handler in /partage/ location block

app/public/admin/fichiers-fragment.php

@@ -1,17 +1,6 @@
 <?php
 /**
- * fichiers-fragment.php (admin)
- *
- * Admin-gated HTMX fragment: returns the combined Format(s) + Fichiers block
- * for the admin add/edit forms.  Wraps the shared logic in fichiers-fragment.php
- * after enforcing authentication.
+ * Admin fragment: Format(s) + Fichiers block (HTMX partial).
+ * @deprecated Use /admin/fragments/fichiers.php instead.
  */
-require_once __DIR__ . '/../../bootstrap.php';
-require_once __DIR__ . '/../../src/AdminAuth.php';
-App::boot();
-
-AdminAuth::requireLogin();
-
-$_POST['admin_mode'] = '1';
-
-require_once APP_ROOT . '/templates/partials/form/fichiers-fragment.php';
+require_once __DIR__ . '/fragments/fichiers.php';