Plugin keyboard shortcuts: open plugins + plugin-defined custom shortcuts

## Summary
Two related keyboard-shortcut capabilities for plugins.

### A. Host shortcuts to open/focus a plugin
- A combo (e.g. Cmd/Ctrl+Shift+1…9) opens or focuses the Nth installed plugin.
- Decide: open the plugin **sidebar** vs a **tab**, or a single shortcut that opens a searchable **plugin picker / command palette** (scales past 9).
- Model on the existing `SwitchToTab` pattern: `ShortcutAction` variant in `src/app/shortcut_handler.rs`, detect combo, dispatch in `thoth_app.rs::handle_shortcut_actions`, enumerate via `PluginManager::get_*_plugins`.

### B. Plugin-defined custom shortcuts (the bigger piece)
- A plugin can **register its own key combos** (e.g. `cmd+enter` → execute request); plugins can set any shortcut.
- When the combo fires while that plugin's tab/sidebar is active, the host dispatches it to the plugin as an event (e.g. a `ui-event` with a shortcut id) so the plugin reacts.
- Requires WIT additions (a way for a plugin to declare/register shortcuts), host-side registration + dispatch, and conflict handling with built-in shortcuts.

## Notes
Split out of the post-#80 request-UI polish batch; needs its own design before implementation.