Add right-click context menu and "..." dropdown button for folders in
the file tree, enabling file operations within any folder:
- New File: Create empty file via Blob upload
- New Folder: Create subfolder
- Upload File: Upload multiple files to folder
- Upload Folder: Upload entire folder structure preserving hierarchy
Implementation includes:
- FileOperationsMenu: Shared menu component for both triggers
- FileTreeContextMenu: Right-click menu with absolute positioning
- FileTreeNode: Added context menu and dropdown button for folders
- Store slice for context menu state management
- i18n strings for en-US and zh-Hans
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