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/common/stubs.rs

@@ -19,7 +19,7 @@ use crate::application::dtos::folder_dto::{
     CreateFolderDto, FolderDto, MoveFolderDto, RenameFolderDto,
 };
 use crate::application::dtos::pagination::{PaginatedResponseDto, PaginationRequestDto};
-use crate::application::dtos::search_dto::{SearchCriteriaDto, SearchResultsDto};
+use crate::application::dtos::search_dto::{SearchCriteriaDto, SearchResultsDto, SearchSuggestionsDto};
 use crate::application::ports::compression_ports::{CompressionLevel, CompressionPort};
 use crate::application::ports::file_ports::{
     FileManagementUseCase, FileRetrievalUseCase, FileUploadUseCase, FileUseCaseFactory,
@@ -250,6 +250,14 @@ impl FolderRepository for StubFolderStoragePort {
         Ok(Vec::new())
     }
 
+    async fn list_folders_by_owner(
+        &self,
+        _parent_id: Option<&str>,
+        _owner_id: &str,
+    ) -> Result<Vec<Folder>, DomainError> {
+        Ok(Vec::new())
+    }
+
     async fn list_folders_paginated(
         &self,
         _parent_id: Option<&str>,
@@ -350,6 +358,14 @@ impl FolderUseCase for StubFolderUseCase {
         Ok(Vec::new())
     }
 
+    async fn list_folders_for_owner(
+        &self,
+        _parent_id: Option<&str>,
+        _owner_id: &str,
+    ) -> Result<Vec<FolderDto>, DomainError> {
+        Ok(Vec::new())
+    }
+
     async fn list_folders_paginated(
         &self,
         _parent_id: Option<&str>,
@@ -559,6 +575,18 @@ impl SearchUseCase for StubSearchUseCase {
         Ok(SearchResultsDto::empty())
     }
 
+    async fn suggest(
+        &self,
+        _query: &str,
+        _folder_id: Option<&str>,
+        _limit: usize,
+    ) -> Result<SearchSuggestionsDto, DomainError> {
+        Ok(SearchSuggestionsDto {
+            suggestions: Vec::new(),
+            query_time_ms: 0,
+        })
+    }
+
     async fn clear_search_cache(&self) -> Result<(), DomainError> {
         Ok(())
     }