fix(security): patch 3 vulnerabilities — IDOR, ownership bypass, XSS

V1: Add owner-scoped folder pagination (list_folders_by_owner_paginated)
  - New method in FolderRepository trait, PG implementation, service & handler
  - Prevents IDOR by filtering folder listings to authenticated user

V2: Enforce ownership checks on folder mutations
  - rename_folder, move_folder, delete_folder now require caller_id
  - Service verifies folder.owner_id == caller_id (returns 404 on mismatch)
  - Propagated to folder_handler, batch_handler, batch_operations, webdav_handler
  - delete_folder_with_trash upgraded from OptionalAuthUser to AuthUser
  - download_folder_zip now checks ownership before streaming

V3: Fix XSS in frontend via DOM APIs
  - sharedView.js: innerHTML → createElement + textContent
  - contextMenus.js: innerHTML → DOM construction for share dialog

Cleanup: removed unused OptionalAuthUser import, updated all stubs/mocks

src/application/services/trash_service_test.rs

@@ -373,6 +373,17 @@ impl FolderRepository for MockFolderRepository {
         Ok((vec![], Some(0)))
     }
 
+    async fn list_folders_by_owner_paginated(
+        &self,
+        _parent_id: Option<&str>,
+        _owner_id: &str,
+        _offset: usize,
+        _limit: usize,
+        _include_total: bool,
+    ) -> std::result::Result<(Vec<Folder>, Option<usize>), DomainError> {
+        Ok((vec![], Some(0)))
+    }
+
     async fn rename_folder(
         &self,
         _id: &str,