fix: briefing TTS now uses saved voice/speed/blend settings

synthesiseSpeech() called without explicit params now omits voice/speed/
blend from the request body. The backend detects this and auto-loads all
three from the user's saved settings (voice_tts_voice, voice_tts_speed,
voice_tts_blend), so briefing listen mode respects the voice the user
configured in Settings.

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

frontend/src/api/client.ts

@@ -666,11 +666,15 @@ export async function synthesiseSpeech(
   speed?: number,
   voiceBlend?: VoiceBlendEntry[]
 ): Promise<Blob> {
-  const body: Record<string, unknown> = { text, speed: speed ?? 1.0 }
+  // Only send voice/speed/blend when explicitly provided — omitting them lets
+  // the server auto-load the user's saved voice settings (voice, speed, blend).
+  const body: Record<string, unknown> = { text }
   if (voiceBlend && voiceBlend.length >= 2) {
     body.voice_blend = voiceBlend
-  } else {
+    if (speed !== undefined) body.speed = speed
+  } else if (voice !== undefined || speed !== undefined) {
     body.voice = voice ?? 'af_heart'
+    body.speed = speed ?? 1.0
   }
   const res = await fetch('/api/voice/synthesise', {
     method: 'POST',