[Feature]: wire the precision button to 0x2205 pointer scaling on devices without adjustable DPI

### Problem / motivation

On the original MX Ergo the precision button (CID `0xed`) is mapped to `ButtonId::DpiToggle`, whose default action is `CycleDpiPresets`. The device has no `0x2201`/`0x2202`, so no DPI presets can ever be configured for it and every press is dropped with `no DPI presets configured for active device — press ignored`. Options+ drives precision mode on this device through `0x2205` PointerMotionScaling instead.

#1347 adds the `0x2205` protocol layer (read/write plus `openlogi diag pointer-speed`), verified on hardware. What is still missing is the product layer: nothing in the config, agent or GUI can use it.

### Proposed solution

Treat pointer scaling as its own capability rather than as a DPI preset:

- A `pointer_scaling` capability flag driven by the presence of `0x2205`, like the existing `0x2121`-driven wheel flags.
- Config: a per-device precision scaling value (the multiplier applied while precision mode is engaged) plus the device's normal scaling.
- Actions: a momentary "precision mode" (scaling applied while held) and/or a toggle, so `0xed` finally does something on devices without adjustable DPI.
- GUI: a slider on the pointer panel for devices that expose `0x2205` but not `0x2201`/`0x2202`.

### Important constraint

The spec says firmware validates and clips out-of-range scaling values. An MX Ergo (`MPM06.03_B0022`) does not: it stored both `0x0001` (pointer nearly frozen) and `0xffff` (256x) unchanged. So whatever exposes this to users must bound the value itself — Solaar uses `0x002e`–`0x01ff`. A UI that writes an unbounded value can leave a user unable to move the pointer to fix it.

### Alternatives considered

Synthesizing fake DPI presets from `0x2205` would reuse the existing DPI plumbing, but the two features are not the same thing: DPI is an absolute sensor resolution, `0x2205` a host-side multiplier. Conflating them would put wrong numbers in the GUI and in `config.toml`.

### Related

- #1347 (protocol layer, open)
- #367 (MX Ergo device support)

Happy to implement this once there is a decision on the config/action shape; I have the hardware to verify it.

Start by reviewing the #1347 protocol layer and the existing 0x2121-driven wheel capability paths across config, agent, and GUI. Decide the config and action shape for 0x2205 pointer scaling, including bounded values and precision-mode behavior. Done means devices with 0x2205 can expose and control precision scaling without being represented as DPI presets.

Written by the indexing model from the issue text.