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/templates/partials/form/form.php

@@ -48,6 +48,10 @@
  *   ?string $contactPublic  — contact visibility flag for edit mode
  *   ?string $contactInterne — contact email for edit mode
  *
+ * Autosave:
+ *   string $formExtraAttrs — extra HTML attributes to inject into the <form> tag
+ *   bool   $showAutosaveStatus — render the "Brouillon enregistré" status indicator
+ *
  * Website:
  *   string $existingWebsiteUrl
  *   string $existingWebsiteLabel
@@ -56,6 +60,8 @@
 
 // ── Defaults ──────────────────────────────────────────────────────────────────
 $mode                    = $mode            ?? 'add';
+$formExtraAttrs          = $formExtraAttrs  ?? '';
+$showAutosaveStatus      = $showAutosaveStatus ?? false;
 // In admin add/edit, no field is required (admins can save partial records)
 $adminMode               = ($mode === 'add' || $mode === 'edit');
 $formAction              = $formAction      ?? '';
@@ -146,7 +152,7 @@ $errorFieldName = $errorFieldName ?? null;
     <?php endif; ?>
 <?php endif; ?>
 
-    <form action="<?= $formAction ?>" method="post" enctype="multipart/form-data" class="admin-form" data-beforeunload-guard>
+    <form action="<?= $formAction ?>" method="post" enctype="multipart/form-data" class="admin-form" data-beforeunload-guard <?= $formExtraAttrs ?>>
         <!-- Default: JS-disabled mode (disabled → not submitted → server uses $_FILES path).
              On DOMContentLoaded, JS enables this input and sets value="1" → server uses FilePond path. -->
         <input type="hidden" name="filepond_mode" value="0" disabled>
@@ -540,6 +546,10 @@ if ($filesMode === 'add'): ?>
         </fieldset>
 <?php endif; ?>
 
+        <?php if ($showAutosaveStatus): ?>
+        <div class="autosave-status" data-autosave-status></div>
+        <?php endif; ?>
+
         <div class="form-footer admin-form-footer">
             <button type="submit" name="go" class="btn btn--primary"><?= $mode === 'edit' ? 'Enregistrer' : 'Soumettre' ?></button>
 <?php if ($mode === 'add' || $mode === 'edit'): ?>