Add autosave draft system for partage form with HTMX-based session persistence

- New fragment endpoint POST/GET /partage/fragments/draft.php:
  saves all form fields to PHP session, excludes file/csrf/slug fields
  GET returns JSON for JS hydration on page load
  rotates both global CSRF and share CSRF tokens in sync

- form.php accepts optional $formExtraAttrs and $showAutosaveStatus:
  allows injecting HTMX attributes and 'Brouillon enregistré' indicator

- renderShareLinkForm adds hx-post with change/input debounce trigger,
  loads autosave-handler.js, hydrate fields from draft on page load

- Draft cleared on successful form submission in handleShareLinkSubmission

- autosave-handler.js now also updates share_link_token hidden input
  when rotating CSRF token (partage form uses both csrf_token and share_link_token)

- Added .autosave-status CSS to form.css (was admin.css-only)

- Updated fragment routing to accept GET requests (needed for draft hydration)

app/src/FilepondHandler.php

@@ -146,7 +146,7 @@ class FilepondHandler
 
         // Track temp file in session so it survives page reloads
         if (session_status() === PHP_SESSION_ACTIVE) {
-            $_SESSION['filepond_tmp'][$queueType] = $_SESSION['filepond_tmp'][$queueType] ?? [];
+            $_SESSION['filepond_tmp'][$queueType] ??= [];
             $_SESSION['filepond_tmp'][$queueType][] = $fileId;
         }