Grab and Move - Allow binding to Right Alt (AltGr) only

### Description of the new feature / enhancement

Currently, the default trigger key for *Grab and Move* is `Alt`, which applies to both left and right Alt keys. This can interfere with standard workflows such as using `Alt + Tab` to switch windows, where holding `Alt` may unintentionally activate Grab and Move and prevent normal window selection with the mouse.

It would be highly beneficial to allow users to configure the trigger key more precisely, specifically enabling an option to bind the feature to **Right Alt (AltGr) only**, while ignoring Left Alt. This would significantly reduce accidental activations during common keyboard shortcuts.

### Scenario when this would be used?

When switching windows using `Alt + Tab`, users often keep holding `Alt` while selecting a window with the mouse. With the current behavior, this may trigger Grab and Move unintentionally, disrupting the workflow.

By restricting activation to Right Alt only, users can:

* Continue using Left Alt for standard OS shortcuts without interference
* Use Right Alt deliberately for Grab and Move interactions
* Avoid conflicts between window management and window switching behaviors

### Supporting information

A similar tool, AltDrag ([https://github.com/stefansundin/altdrag](https://github.com/stefansundin/altdrag)), already supports binding the functionality exclusively to Right Alt, demonstrating both feasibility and practical value of this feature.

Start by locating the Grab and Move trigger-key configuration and keyboard input handling. Confirm how the current Alt binding treats left and right Alt, then define the configuration change so Right Alt can activate Grab and Move while Left Alt does not. Done means the option works without disrupting standard Alt+Tab use.

Written by the indexing model from the issue text.