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/share_service.rs

@@ -544,6 +544,18 @@ mod tests {
             unimplemented!()
         }
 
+        async fn list_folders_by_owner_paginated(
+            &self,
+            _parent_id: Option<&str>,
+            _owner_id: &str,
+            _offset: usize,
+            _limit: usize,
+            _include_total: bool,
+        ) -> Result<(Vec<crate::domain::entities::folder::Folder>, Option<usize>), DomainError>
+        {
+            unimplemented!()
+        }
+
         async fn rename_folder(
             &self,
             _id: &str,