Greeter/DankGreet: Fingerprint scan animation

### Feature Description

It would be nice to have a visual animation on the fingerprint icon area during fingerprint auth. [Something like this](https://lottiefiles.com/free-animation/unlock-wq6TJS7SGL) where the moving rings rotate while the sensor is scanning, green checkmark on success, red X on failure.

Right now the greeter is totally static during fingerprint reads. There is no indication anything is happening until it either unlocks or silently fails.

### Use Case

It is mostly to make the greeter more dynamic but it also provides clear feedback when the reader fails instead of leaving the user guessing.
Compositor(s): All compositors

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

- Scan active → animation in place of the fingerprint icon ([rings moving](https://lottiefiles.com/free-animation/unlock-wq6TJS7SGL), [color changing](https://lottiefiles.com/marketplace/fingerprint-scan-animation-pack_192382), [fingerprint getting drawn](https://lottiefiles.com/marketplace/biometric-fingerprint-animation-pack_385218))
- Success → green check, proceed to session
- Failure → red X, reset to idle

Ideally the animation/icon style would be configurable in settings so users can pick which scan animation they prefer.

The greeter already listens to fprintd's D-Bus signals for auth — this would just be an animation layer on top.

### Alternatives/Existing Solutions

None that I am aware of for greetd.

Start by locating DankGreet’s fingerprint icon area and the existing fprintd D-Bus authentication signal handling. Trace the idle, scanning, success, and failure states before choosing how the animation layer fits across all compositors. Done means visible scan feedback, a green success state, a red failure reset, and tests or manual checks for each state.

Written by the indexing model from the issue text.