feat: folder ownership scoping, batch operations integration, frontend audit fixes

Backend:
- Add owner_id to Folder entity + FolderDto (DB user_id column)
- Add list_folders_by_owner to FolderRepository trait + PG impl
- Add list_folders_for_owner to FolderUseCase + FolderService
- Rewrite FolderHandler: all endpoints now scope by AuthUser
- Remove dead handler methods (list_folders_inner, list_folders_for_user, is_user_home_folder, folder_belongs_to_user)
- Add ownership check in get_folder (returns 404 on mismatch)

Batch operations:
- Add trash_service + zip_service to BatchOperationService
- New methods: trash_files, trash_folders, move_folders, download_zip
- New handlers: trash_batch, move_folders_batch, download_batch
- New routes: POST /api/batch/trash, /api/batch/folders/move, /api/batch/download

Frontend:
- Replace findUserHomeFolder (~130 lines) with resolveHomeFolder (~35 lines)
- Remove client-side folder filtering in loadFiles (backend now scopes)
- Rewrite batchDelete: N requests -> 1 POST /api/batch/trash
- Rewrite batchMove: N requests -> 2 POST max (files + folders)
- Rewrite batchDownload: N requests -> 1 POST /api/batch/download (ZIP)
- Search moved to backend, share system uses backend API
- Dark mode fixes, frontend audit improvements

src/application/services/folder_service.rs

@@ -45,6 +45,14 @@ impl FolderService {
                 Ok(vec![])
             }
 
+            async fn list_folders_for_owner(
+                &self,
+                _parent_id: Option<&str>,
+                _owner_id: &str,
+            ) -> Result<Vec<FolderDto>, DomainError> {
+                Ok(vec![])
+            }
+
             async fn list_folders_paginated(
                 &self,
                 _parent_id: Option<&str>,
@@ -173,6 +181,29 @@ impl FolderUseCase for FolderService {
         Ok(folders.into_iter().map(FolderDto::from).collect())
     }
 
+    /// Lists folders scoped to a specific owner.
+    async fn list_folders_for_owner(
+        &self,
+        parent_id: Option<&str>,
+        owner_id: &str,
+    ) -> Result<Vec<FolderDto>, DomainError> {
+        let folders = self
+            .folder_storage
+            .list_folders_by_owner(parent_id, owner_id)
+            .await
+            .map_err(|e| {
+                DomainError::internal_error(
+                    "FolderStorage",
+                    format!(
+                        "Failed to list folders for owner '{}' in parent {:?}: {}",
+                        owner_id, parent_id, e
+                    ),
+                )
+            })?;
+
+        Ok(folders.into_iter().map(FolderDto::from).collect())
+    }
+
     /// Lists folders with pagination
     async fn list_folders_paginated(
         &self,