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/share_dto.rs

@@ -6,6 +6,7 @@ use crate::domain::entities::share::{Share, SharePermissions};
 pub struct ShareDto {
     pub id: String,
     pub item_id: String,
+    pub item_name: Option<String>,
     pub item_type: String,
     pub token: String,
     pub url: String,
@@ -27,6 +28,7 @@ pub struct SharePermissionsDto {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct CreateShareDto {
     pub item_id: String,
+    pub item_name: Option<String>,
     pub item_type: String,
     pub password: Option<String>,
     pub expires_at: Option<u64>,
@@ -48,6 +50,7 @@ impl ShareDto {
         Self {
             id: share.id().to_string(),
             item_id: share.item_id().to_string(),
+            item_name: share.item_name().map(|s| s.to_string()),
             item_type: share.item_type().to_string(),
             token: share.token().to_string(),
             url,