Add keyboard shortcut system to xserial-gui

Introduce a centralized shortcut system with ShortcutAction enum and bindings table in shortcuts.rs. Global shortcuts are processed at the top of XserialApp::ui() before any widget rendering. Text input focus is detected via wants_keyboard_input() to suppress shortcuts when typing. Also fix pre-existing import ordering in panels/config.rs from cargo fmt.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

crates/xserial-gui/src/main.rs

@@ -3,6 +3,7 @@ mod app_state;
 mod buffers;
 mod panels;
 mod perf;
+mod shortcuts;
 mod ui_fonts;
 
 use xserial_client::SessionManager;