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

static/css/auth.css

@@ -465,3 +465,121 @@
         max-height: 200px;
     }
 }
+
+/* ============================================================
+   DARK MODE — Auth Pages
+   ============================================================ */
+[data-theme="dark"] .auth-container {
+    background-color: #0f172a;
+}
+[data-theme="dark"] .auth-panel {
+    background-color: #1e293b;
+    box-shadow: 0 5px 20px rgba(0, 0, 0, 0.3);
+}
+[data-theme="dark"] .auth-logo-text {
+    color: #f1f5f9;
+}
+[data-theme="dark"] .auth-title {
+    color: #f1f5f9;
+}
+[data-theme="dark"] .auth-label {
+    color: #94a3b8;
+}
+[data-theme="dark"] .auth-input {
+    background-color: #0f172a;
+    border-color: #334155;
+    color: #e2e8f0;
+}
+[data-theme="dark"] .auth-input:focus {
+    border-color: #ff5e3a;
+    background-color: #0f172a;
+    box-shadow: 0 0 0 3px rgba(255, 94, 58, 0.15);
+}
+[data-theme="dark"] .auth-input::placeholder {
+    color: #64748b;
+}
+[data-theme="dark"] .auth-error {
+    background-color: #3b1111;
+    color: #fca5a5;
+}
+[data-theme="dark"] .auth-success {
+    background-color: #052e16;
+    color: #86efac;
+}
+[data-theme="dark"] .auth-toggle {
+    color: #94a3b8;
+}
+[data-theme="dark"] .auth-divider {
+    color: #64748b;
+}
+[data-theme="dark"] .auth-divider::before,
+[data-theme="dark"] .auth-divider::after {
+    background: #334155;
+}
+[data-theme="dark"] .language-subtitle {
+    color: #94a3b8;
+}
+[data-theme="dark"] .lang-picker-selected {
+    background-color: #0f172a;
+    border-color: #334155;
+}
+[data-theme="dark"] .lang-picker-selected:hover {
+    border-color: #ff5e3a;
+    background-color: #162032;
+}
+[data-theme="dark"] .lang-picker.open .lang-picker-selected {
+    border-color: #ff5e3a;
+    background-color: #162032;
+}
+[data-theme="dark"] .lang-picker-name {
+    color: #f1f5f9;
+}
+[data-theme="dark"] .lang-picker-arrow {
+    color: #64748b;
+}
+[data-theme="dark"] .lang-picker-dropdown {
+    background: #1e293b;
+    border-color: #ff5e3a;
+    border-top-color: #334155;
+    box-shadow: 0 12px 32px rgba(0, 0, 0, 0.3);
+}
+[data-theme="dark"] .lang-picker-search {
+    border-bottom-color: #334155;
+}
+[data-theme="dark"] .lang-picker-search input {
+    background-color: #0f172a;
+    border-color: #334155;
+    color: #e2e8f0;
+}
+[data-theme="dark"] .lang-picker-search input:focus {
+    border-color: #ff5e3a;
+}
+[data-theme="dark"] .lang-picker-search input::placeholder {
+    color: #64748b;
+}
+[data-theme="dark"] .lang-picker-list::-webkit-scrollbar-thumb {
+    background: #475569;
+}
+[data-theme="dark"] .lang-picker-item:hover {
+    background: #162032;
+}
+[data-theme="dark"] .lang-picker-item.selected {
+    background: #2a1a15;
+}
+[data-theme="dark"] .lang-picker-item-name {
+    color: #f1f5f9;
+}
+[data-theme="dark"] .lang-picker-item-english {
+    color: #64748b;
+}
+[data-theme="dark"] .lang-picker-empty {
+    color: #64748b;
+}
+/* Setup steps */
+[data-theme="dark"] .step-number {
+    background-color: #334155;
+    color: #94a3b8;
+}
+[data-theme="dark"] .step-title {
+    color: #94a3b8;
+}