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/fragments/fichiers.php

@@ -0,0 +1,13 @@
+<?php
+/**
+ * Admin fragment: Format(s) + Fichiers block (HTMX partial).
+ */
+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';