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/screens/chat/chat_screen.dart

@@ -267,6 +267,7 @@ class _ChatScreenState extends ConsumerState<ChatScreen>
                   VoiceMicButton(
                     mode: voiceState.mode,
                     voiceModeActive: voiceState.voiceModeActive,
+                    amplitude: voiceState.amplitude,
                     onTap: _toggleVoiceMode,
                   ),
                   const SizedBox(width: 6),