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/domain/repositories/folder_repository.rs

@@ -54,6 +54,18 @@ pub trait FolderRepository: Send + Sync + 'static {
         include_total: bool,
     ) -> Result<(Vec<Folder>, Option<usize>), DomainError>;
 
+    /// Lists folders with pagination, scoped to a specific owner.
+    /// Combines the owner filtering of `list_folders_by_owner` with
+    /// the pagination of `list_folders_paginated`.
+    async fn list_folders_by_owner_paginated(
+        &self,
+        parent_id: Option<&str>,
+        owner_id: &str,
+        offset: usize,
+        limit: usize,
+        include_total: bool,
+    ) -> Result<(Vec<Folder>, Option<usize>), DomainError>;
+
     /// Renames a folder
     async fn rename_folder(&self, id: &str, new_name: String) -> Result<Folder, DomainError>;