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/providers/voice_provider.dart

@@ -59,22 +59,29 @@ class VoiceState {
   final VoiceMode mode;
   final bool voiceModeActive;
   final bool available;
+  /// Normalized mic amplitude 0.0–1.0 while recording. Drives the live
+  /// pulse on VoiceMicButton so the user has obvious feedback that audio
+  /// is actually being picked up.
+  final double amplitude;
 
   const VoiceState({
     this.mode = VoiceMode.idle,
     this.voiceModeActive = false,
     this.available = true,
+    this.amplitude = 0.0,
   });
 
   VoiceState copyWith({
     VoiceMode? mode,
     bool? voiceModeActive,
     bool? available,
+    double? amplitude,
   }) =>
       VoiceState(
         mode: mode ?? this.mode,
         voiceModeActive: voiceModeActive ?? this.voiceModeActive,
         available: available ?? this.available,
+        amplitude: amplitude ?? this.amplitude,
       );
 }
 
@@ -267,14 +274,24 @@ class VoiceNotifier extends Notifier<VoiceState> {
   void _onAmplitude(Amplitude event) {
     if (!state.voiceModeActive) return;
 
+    final db = event.current;
+    final validDb = !(db.isNaN || db.isInfinite);
+
+    // Normalize dB to 0..1 for the pulse animation. -60dB is dead-quiet,
+    // 0dB is peak; we clamp and bias so the button visibly breathes even
+    // on soft speech without exploding on loud input.
+    if (validDb) {
+      final norm = ((db + 60.0) / 60.0).clamp(0.0, 1.0);
+      if ((norm - state.amplitude).abs() > 0.02) {
+        state = state.copyWith(amplitude: norm);
+      }
+    }
+
     final elapsed =
         DateTime.now().millisecondsSinceEpoch - _recordingStartMs;
     if (elapsed < _minRecordingMs) return;
 
-    final db = event.current;
-    // Guard against NaN / ±Infinity which can arrive on some Android devices
-    // when the recorder is initialising. Treat invalid readings as silence.
-    final isSilent = db.isNaN || db.isInfinite || db < _silenceThresholdDb;
+    final isSilent = !validDb || db < _silenceThresholdDb;
 
     if (isSilent) {
       _silenceMs += 200;