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/dtos/folder_dto.rs

@@ -40,6 +40,10 @@ pub struct FolderDto {
     /// Parent folder ID
     pub parent_id: Option<String>,
 
+    /// Owner user ID (scopes visibility per user)
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub owner_id: Option<String>,
+
     /// Creation timestamp
     pub created_at: u64,
 
@@ -59,6 +63,7 @@ impl From<Folder> for FolderDto {
             name: folder.name().to_string(),
             path: folder.path_string().to_string(),
             parent_id: folder.parent_id().map(String::from),
+            owner_id: folder.owner_id().map(String::from),
             created_at: folder.created_at(),
             modified_at: folder.modified_at(),
             is_root,
@@ -90,6 +95,7 @@ impl FolderDto {
             name: "stub-folder".to_string(),
             path: "/stub/path".to_string(),
             parent_id: None,
+            owner_id: None,
             created_at: 0,
             modified_at: 0,
             is_root: true,