Make idle head motion configurable in AI Agent mode

[rebelthor]

Summary

IdleMotionModifier runs randomized head motion while the AI Agent is in STANDBY. Cadence and motion travel are hardcoded today.

Implemented a small on-device "Idle Behavior" screen under Setup → AI.Agent so users can quiet or disable it without rebuilding firmware. Validated on hardware (CoreS3).

Controls

Control	Notes
Enable	Toggle. Off = modifier not added.
Frequency	Sparse / Normal / Frequent. Normal preserves the current 4-8 sec interval.
Intensity	Calm / Normal / Lively. Scales motion travel only, not speed (otherwise long idle motions starve HeadPetModifier).

Defaults preserve current behavior. Settings persist in NVS via the
existing XiaozhiConfig_t / Settings pattern.

[SeveranExp]

lol perfect, I created a feature request but this is pretty much the same thing, love it!