feat: add hotkeys, allow active preset to be changed by keypress

[OpenSauce]

Closes #79

[Copilot]

Pull request overview

Adds configurable keyboard hotkeys to load presets from the GUI, including a dedicated Hotkey Settings dialog and persisted mappings in the app settings.

Changes:

• Persist hotkey mappings in Settings and expose a new hotkey module with (de)serialization + match logic.
• Add a Hotkey Settings dialog + handler and wire new GUI messages for managing mappings.
• Subscribe to global keyboard events and trigger preset selection when a mapping matches.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/settings/mod.rs	Adds HotkeySettings to persisted Settings with serde defaults.
src/lib.rs	Exposes new hotkey module.
src/i18n/mod.rs	Adds EN/ZH-CN translation strings for the hotkey dialog.
src/hotkey/mod.rs	Implements hotkey mapping model, serialization helpers, and matching logic.
src/gui/messages/mod.rs	Adds hotkey-related messages and a KeyPressed message.
src/gui/messages/hotkey.rs	Defines HotkeyMessage enum for dialog interactions.
src/gui/handlers/mod.rs	Registers the new hotkey handler module.
src/gui/handlers/hotkey.rs	Introduces HotkeyHandler to manage dialog state + settings updates.
src/gui/components/dialogs/mod.rs	Registers the new hotkey dialog module.
src/gui/components/dialogs/hotkey.rs	Implements the Hotkey Settings dialog UI and learning flow.
src/gui/app.rs	Wires the handler/dialog into the app, adds keyboard subscription, triggers preset changes on hotkeys.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.