[Feature]: Support Shift + Main Wheel for Horizontal Scrolling

### Pre-flight checklist

- [x] I searched existing issues and the [Roadmap](https://github.com/AprilNEA/OpenLogi#roadmap), and this isn't already tracked.

### Problem / motivation

## Description

It would be useful to support using the **Shift key + main scroll wheel** to perform horizontal scrolling on macOS.

### Expected behavior

| Input | Expected action |
|---|---|
| Wheel Up | Vertical scroll up |
| Wheel Down | Vertical scroll down |
| **Shift + Wheel Up** | **Horizontal scroll right** |
| **Shift + Wheel Down** | **Horizontal scroll left** |

This is a common scrolling behavior in macOS applications and would be especially useful with the **MX Master 3S**.

## Current behavior

OpenLogi supports horizontal scrolling through the thumb wheel, for example:

```toml
[devices."serial:...".bindings]
ThumbwheelScrollUp = "HorizontalScrollRight"
ThumbwheelScrollDown = "HorizontalScrollLeft"
```
However, there does not appear to be a way to configure the main wheel + Shift modifier through the TOML configuration.

### Proposed solution

## Proposed configuration

Ideally, this could be configured in TOML, for example:

```toml
[devices."serial:...".bindings]
ShiftScrollUp = "HorizontalScrollRight"
ShiftScrollDown = "HorizontalScrollLeft"
```

The exact configuration syntax can follow the existing OpenLogi design.

### Related area(s)

- [x] GUI
- [ ] CLI
- [ ] Button actions / remapping
- [ ] DPI
- [ ] SmartShift
- [ ] Per-application profiles
- [x] Configuration (TOML)
- [ ] Auto-update
- [ ] Other

### Additional context

The desired behavior would be particularly useful for users who want to keep the thumb wheel assigned to another function while using **Shift + main wheel** for horizontal scrolling.

Thank you for considering this feature.

_No response_

Start with the existing TOML bindings for ThumbwheelScrollUp and ThumbwheelScrollDown and trace how wheel input and modifiers are represented. Confirm the existing main-wheel vertical behavior, then follow the project's established configuration syntax; done means Shift + Wheel Up/Down produce horizontal right/left on macOS while ordinary wheel behavior remains unchanged.

Written by the indexing model from the issue text.