feat: Kokoro voice blending — blend builder UI + weighted tensor synthesis

Add voice blend support to TTS pipeline and settings UI. Users can mix
2–5 Kokoro voices with per-voice weight sliders; the blended style tensor
replaces the single voice when enabled. Settings persist as JSON and auto-
load on synthesis when no explicit voice is supplied in the request.

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

frontend/src/api/client.ts

@@ -639,6 +639,11 @@ export interface VoiceEntry {
   label: string
 }
 
+export interface VoiceBlendEntry {
+  voice: string
+  weight: number
+}
+
 export const getVoiceStatus = () => apiGet<VoiceStatusResult>('/api/voice/status')
 
 export const getVoiceList = () =>
@@ -658,12 +663,19 @@ export async function transcribeAudio(blob: Blob): Promise<{ transcript: string;
 export async function synthesiseSpeech(
   text: string,
   voice?: string,
-  speed?: number
+  speed?: number,
+  voiceBlend?: VoiceBlendEntry[]
 ): Promise<Blob> {
+  const body: Record<string, unknown> = { text, speed: speed ?? 1.0 }
+  if (voiceBlend && voiceBlend.length >= 2) {
+    body.voice_blend = voiceBlend
+  } else {
+    body.voice = voice ?? 'af_heart'
+  }
   const res = await fetch('/api/voice/synthesise', {
     method: 'POST',
     headers: { 'Content-Type': 'application/json' },
-    body: JSON.stringify({ text, voice: voice ?? 'af_heart', speed: speed ?? 1.0 }),
+    body: JSON.stringify(body),
   })
   if (!res.ok) {
     const err = await res.json().catch(() => ({ error: `HTTP ${res.status}` }))