[Feature]: DPI 'focus mode'

### 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

reimplement a feature that existed in option 'non-plus' and that they dropped

### Proposed solution

Add an Action Binding for Holding a button:
Define Action on Press and Action on Release
in this case
1. StoreCurrentDPI
1. onPress->SetDpi to value
1. OnRelease->RestoreDPI

but the idea is that it could be extended to macros and other bindings
In the UI it might b best to add a sidebar for advanced configuration

like holding the button and sliding volume / DPI value on set axis

### Alternatives considered

_No response_

### Related area(s)

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

### Additional context

Core, Action, DPI, Mappings

Start by locating the Core, Action, DPI, and Mappings entry points and tracing how button actions are configured through the GUI, CLI, and TOML. Define the press/release lifecycle, stored and restored DPI behavior, and how the action can later support macros or other bindings; done means the focus-mode action is configurable and works as proposed.

Written by the indexing model from the issue text.