[Feature]: Allow mapping custom keystrokes/characters (e.g., German umlauts, hashtags) to the thumb wheel

### 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

I would like to have the ability to map specific individual keystrokes, custom characters, and modifiers to the lateral thumb wheel. Currently, it lacks the flexibility to assign exact single-character inputs.

In applications like **Adobe Photoshop**, the brush size adjustment is hardcoded to specific keys depending on the keyboard layout. For example, on a German keyboard layout, changing the brush size requires the **hashtag (#)** and **plus (+)** or **ö/ä** keys.

### Proposed solution

If OpenLogi allows mapping the thumb wheel to these specific German umlauts and symbols, users could seamlessly control brush sizes or zoom increments in creative software just by rotating the wheel.

### Alternatives considered

### Describe alternatives you've considered
Using external remapping tools (like AutoHotkey or Karabiner), but having native support inside OpenLogi would be much cleaner and more reliable for cross-platform workflows.

### Related area(s)

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

### Additional context

This feature would greatly benefit international users who rely on non-US keyboard layouts for their application shortcuts.

No files or tests are named. Start by tracing the GUI's thumb-wheel mapping and TOML configuration entry points; done means users can assign individual characters, symbols such as # and +, and modifiers to the lateral wheel, including German umlauts, without relying on external remapping tools.

Written by the indexing model from the issue text.