fix(skill): improve accessibility for file tree and tabs

- Convert div with onClick to proper button elements for keyboard access
- Add focus-visible ring styles to all interactive elements
- Add ARIA attributes (role, aria-selected, aria-expanded) to tree nodes
- Add keyboard navigation (Enter/Space) support to tree items
- Mark decorative icons with aria-hidden="true"
- Add missing i18n keys for accessibility labels
- Fix typography: use ellipsis character (…) instead of three dots

web/i18n/en-US/workflow.json

@@ -1018,6 +1018,7 @@
   "skillSidebar.menu.filesUploaded": "{{count}} file(s) uploaded successfully",
   "skillSidebar.menu.folderCreated": "Folder created successfully",
   "skillSidebar.menu.folderUploaded": "Folder uploaded successfully",
+  "skillSidebar.menu.moreActions": "More actions",
   "skillSidebar.menu.newFile": "New File",
   "skillSidebar.menu.newFilePrompt": "Enter file name (with extension, e.g., script.py):",
   "skillSidebar.menu.newFolder": "New Folder",
@@ -1028,8 +1029,9 @@
   "skillSidebar.menu.uploadFile": "Upload File",
   "skillSidebar.menu.uploadFolder": "Upload Folder",
   "skillSidebar.newFolder": "New folder",
-  "skillSidebar.searchPlaceholder": "Search files...",
-  "skillSidebar.uploading": "Uploading...",
+  "skillSidebar.searchPlaceholder": "Search files…",
+  "skillSidebar.toggleFolder": "Toggle folder",
+  "skillSidebar.uploading": "Uploading…",
   "tabs.-": "Default",
   "tabs.addAll": "Add all",
   "tabs.agent": "Agent Strategy",