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

Cargo.lock

@@ -1686,7 +1686,7 @@ checksum = "42f5e15c9953c5e4ccceeb2e7382a716482c34515315f7b03532b8b4e8393d2d"
 
 [[package]]
 name = "oxicloud"
-version = "0.3.2"
+version = "0.3.3"
 dependencies = [
  "anyhow",
  "argon2",