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/shared.html

@@ -4,6 +4,8 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>OxiCloud - Shared Resources</title>
+    <!-- Apply saved theme immediately to prevent flash of light mode -->
+    <script>if(localStorage.getItem('oxicloud_theme')==='dark')document.documentElement.setAttribute('data-theme','dark');</script>
     <link rel="stylesheet" href="/css/style.css">
     <link rel="icon" href="/favicon.ico" type="image/x-icon">
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.0.0-beta3/css/all.min.css">
@@ -74,6 +76,13 @@
                             <span class="user-email" id="user-menu-email">admin@oxicloud.local</span>
                         </div>
                         <div class="user-menu-divider"></div>
+                        <div class="user-menu-item" id="menu-theme">
+                            <i class="fas fa-moon"></i>
+                            <span data-i18n="user_menu.appearance">Appearance</span>
+                            <div class="theme-toggle-pill" id="theme-toggle-pill">
+                                <div class="theme-toggle-knob"></div>
+                            </div>
+                        </div>
                         <div class="user-menu-item" id="menu-logout">
                             <i class="fas fa-sign-out-alt"></i>
                             <span data-i18n="actions.logout">Log out</span>
@@ -284,6 +293,21 @@
                     window.location.href = '/login';
                 });
             }
+
+            // Theme toggle (dark mode)
+            const themeBtn = document.getElementById('menu-theme');
+            const pill = document.getElementById('theme-toggle-pill');
+            if (themeBtn && pill) {
+                const isDark = localStorage.getItem('oxicloud_theme') === 'dark';
+                if (isDark) pill.classList.add('active');
+                themeBtn.addEventListener('click', (e) => {
+                    e.stopPropagation();
+                    pill.classList.toggle('active');
+                    const dark = pill.classList.contains('active');
+                    localStorage.setItem('oxicloud_theme', dark ? 'dark' : 'light');
+                    document.documentElement.setAttribute('data-theme', dark ? 'dark' : 'light');
+                });
+            }
         })();
     </script>
     <script src="/js/shared.js"></script>