fix: share dialog not opening + connect to backend API

- Fix showShareDialog: add try-catch, null checks, prevent textContent
  from destroying header icon (use span child instead)
- Capture file/folder target before closeContextMenu to prevent race
- createSharedLink now calls real backend POST /api/shares instead of
  localStorage-only mock (still caches locally for offline compat)
- Fix share_handler.rs: use OptionalAuthUser instead of AuthUser to
  prevent 401 when auth is disabled (same pattern as delete/trash)
- Add null-safety to closeShareDialog
- Reset new-share-section on dialog open

static/js/ui.js

@@ -204,8 +204,8 @@ const ui = {
                 contextMenus.closeShareDialog();
             });
             
-            document.getElementById('share-confirm-btn').addEventListener('click', () => {
-                contextMenus.createSharedLink();
+            document.getElementById('share-confirm-btn').addEventListener('click', async () => {
+                await contextMenus.createSharedLink();
             });
             
             document.getElementById('copy-share-btn').addEventListener('click', async () => {