[Feature]: Native "Minimize active window" button action

### 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

There's no built-in action to minimize just the currently focused window. The only close options are `ShowDesktop` (minimizes *all* windows) and macOS-only `MissionControl`/`AppExpose`. On Windows, `CustomShortcut` mapped to the native "Win+Down" minimize shortcut doesn't work either — see the second issue I'm filing.

### Proposed solution

Add a dedicated `Minimize` (or `MinimizeActiveWindow`) built-in action, implemented natively per platform (e.g. on Windows via `ShowWindow(GetForegroundWindow(), SW_MINIMIZE)`, on macOS via the Accessibility API), so it works out of the box without scripting workarounds.

### Alternatives considered

- `RunShellCommand` with a PowerShell one-liner calling Win32 `ShowWindow`/`GetForegroundWindow` — works, but on Windows the command appears to be spawned via a shell wrapper without `CREATE_NO_WINDOW`, so a console window briefly flashes on every press. Also, this approach is flagged/quarantined by some corporate EDR tools (e.g. SentinelOne) when compiled into a standalone helper .exe, which isn't practical in managed environments.
- `Shell.Application.MinimizeAll()` via COM — minimizes *all* windows, not just the active one, so it doesn't fit the use case.

### Related area(s)

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

### Additional context

Device: Logitech Lift Vertical Ergonomic Mouse, OpenLogi on Windows 11. Would also be worth checking whether `RunShellCommand` sets `CREATE_NO_WINDOW` when spawning the shell wrapper on Windows — it currently doesn't seem to, causing a visible console flash for any RunShellCommand binding.

Start by locating the built-in button-action definitions and the platform-specific window-control entry points in the Rust application. Compare the existing ShowDesktop and macOS actions, then verify how configuration exposes actions in TOML. Done means a Minimize action targets only the focused window on supported platforms and can be triggered through button remapping without minimizing every window.

Written by the indexing model from the issue text.