feat: Phase 3 complete — SFTP sidebar with full file operations
Rust SFTP service: russh-sftp client on same SSH connection,
DashMap storage, list/read/write/mkdir/delete/rename/stat ops.
5MB file size guard. Non-fatal SFTP failure (terminal still works).
Vue frontend: FileTree with all toolbar buttons wired (upload,
download, delete, mkdir, refresh), TransferProgress panel,
useSftp composable with CWD following via Tauri events.
MainLayout wired with SFTP sidebar.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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