[Feature]: Map mouse buttons combinations to actions

### 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

It would be nice to be able to map combinations of mouse buttons (e.g. forward/back button + left/right click) to actual actions/shortcuts/macros, just as it is already possible for single button clicks or gestures.

### Proposed solution

Map buttons combinations instead of single click / gestures only

### Alternatives considered

_No response_

### Related area(s)

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

### Additional context

_No response_

No files, tests, or entry points are named. Start by locating the existing single-button and gesture remapping entry points, then determine how button combinations should be represented and configured; done means combinations can trigger the same actions, shortcuts, or macros without breaking existing mappings.

Written by the indexing model from the issue text.