feat: add built-in file editor with syntax highlighting

- Add CodeMirror 6 editor with support for 40+ languages
- Add file browser sidebar with directory tree navigation
- Add multi-tab support with dirty state indicators
- Add keyboard shortcuts (Ctrl+E, Ctrl+B, Ctrl+S, Ctrl+W)
- Add editor toggle button to status bar
- Editor uses current session's CWD and requires connection
- Add Tauri commands for file operations (list, read, write)

src-tauri/src/lib.rs

@@ -151,6 +151,9 @@ pub fn run() {
             search_clipboard_entries,
             get_clipboard_languages,
             update_clipboard_language,
+            list_directory,
+            read_file_content,
+            write_file_content,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");