feat(voice): pulse mic button with live amplitude

VoiceState now carries a normalized mic amplitude (0..1) updated
from the existing onAmplitudeChanged subscription, with a 0.02
change threshold so we don't spam rebuilds.

VoiceMicButton swaps the constant-rate AnimationController for an
AnimatedScale + animated glow driven by the live amplitude. 0.1
floor keeps the button breathing on silence; 120ms ease-out smooths
between 200ms samples.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/app.dart

@@ -653,6 +653,7 @@ class _QuickCaptureBarState extends ConsumerState<_QuickCaptureBar> {
                 VoiceMicButton(
                   mode: ref.watch(voiceProvider).mode,
                   voiceModeActive: ref.watch(voiceProvider).voiceModeActive,
+                  amplitude: ref.watch(voiceProvider).amplitude,
                   onTap: _toggleCaptureMic,
                 ),
                 IconButton(