fix(perf): reduce idle CPU — gate audio sink, static shadow radius, cancel glow animation

[rdemeritt]

Summary

• Gate audioLevelPublisher sink on isRecording — stops 23–43 @Published updates/sec from AVAudioEngine tap firing while not recording
• Make idle outer shadow radius static (2 pt) — animated radius forced per-frame CPU compositing at display refresh rate; only opacity continues to animate (CA-native)
• Wrap glowPhase = false in withAnimation(.linear(duration: 0)) — explicitly cancels the running repeatForever CAAnimation on recording start, preventing animation accumulation across multiple record/stop cycles

Root cause

Three independent idle CPU leaks:

1. AVAudioEngine tap fires ~23–43 Hz unconditionally; each tick wrote to two @Published properties (audioLevel, sampleBuffer), dirtying the SwiftUI view tree at tap rate even when idle
2. SwiftUI .shadow(radius:) with an animated radius is not CA-native — compositor re-rendered the HUD layer at display refresh rate (60/120 Hz) for the entire app lifetime
3. Setting glowPhase = false without withAnimation left orphaned repeatForever CAAnimations accumulating on the render server across record/stop cycles

Test plan

• Activity Monitor: CPU usage visibly lower while WhisKey idle (not recording)
• Glow border pulse still animates on the idle pill
• Hotkey down → waveform and recording dot appear correctly
• Hotkey release → waveform stops, pill collapses, glow resumes
• Multiple record/stop cycles — no animation degradation over time
• Reduce Motion: no regressions