feat(phase9): register write_text_file_cmd in invoke_handler

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -371,6 +371,8 @@ pub fn run() {
             commands::windows::close_preview_window,
             // Clipboard
             commands::clipboard::copy_to_clipboard,
+            // Filesystem (Phase 9 save-dialog path)
+            commands::fs::write_text_file_cmd,
             // Paste (auto-insert at cursor)
             commands::paste::paste_text,
             commands::paste::paste_text_replacing,