feat: add breadcrumb navigation and refactor SPA view management

- Add breadcrumb navigation with folder hierarchy display
- Create setCurrentSection() helper to centralize view state management
- Derive nav item section dynamically from DOM data-i18n attribute
- Remove remnant /shared page and consolidate to SPA sharedView
- Add search input to sharedView for client-side filtering
- Fix 'Go to Files' button to use switchToFilesView()

static/js/app/state.js

@@ -11,6 +11,7 @@ window.app = {
     contextMenuTargetFile: null,
     selectedTargetFolderId: '',
     moveDialogMode: 'file',
+    isFilesView: true,
     isTrashView: false,
     isSharedView: false,
     isFavoritesView: false,
@@ -20,6 +21,8 @@ window.app = {
     shareDialogItem: null,
     shareDialogItemType: null,
     notificationShareUrl: null,
+    userHomeFolderId: null,
+    userHomeFolderName: null,
     breadcrumbPath: [] // Array of {id, name} tracking folder navigation hierarchy
 };