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/Controllers/ExportController.php

@@ -175,7 +175,7 @@ class ExportController
                     $ptInstanceUrl = $this->getPeerTubeInstanceUrl();
                 }
                 $tid = (int) $f['thesis_id'];
-                $peertubeLinks[$tid] = $peertubeLinks[$tid] ?? ['dirname' => '', 'links' => []];
+                $peertubeLinks[$tid] ??= ['dirname' => '', 'links' => []];
                 $peertubeLinks[$tid]['links'][] = [
                     'uuid'     => $uuid,
                     'url'      => $ptInstanceUrl !== '' ? rtrim($ptInstanceUrl, '/') . '/videos/watch/' . $uuid : '',