feat: add Edge TTS rate/pitch sliders to voice settings (#629)

Add speed (rate) and pitch controls for Edge TTS provider:
- Frontend: speedToEdgeRate()/hzToEdgePitch() helpers + UI sliders
- Backend: rate/pitch passthrough in OpenaiTtsRequest and controller
- i18n: add edgeRate/edgePitch keys across all 8 languages
- Rate: 0.5x-2.0x slider, Pitch: -20Hz to +20Hz slider

packages/client/src/utils/ttsHelpers.ts

@@ -0,0 +1,16 @@
+/**
+ * 语速倍率 → Edge TTS rate 字符串
+ * 1.0 → "0%", 1.2 → "+20%", 0.5 → "-50%"
+ */
+export function speedToEdgeRate(speed: number): string {
+  const percent = Math.round((speed - 1) * 100)
+  return percent >= 0 ? `+${percent}%` : `${percent}%`
+}
+
+/**
+ * Hz 偏移值 → Edge TTS pitch 字符串
+ * 0 → "0Hz", 12 → "+12Hz", -8 → "-8Hz"
+ */
+export function hzToEdgePitch(hz: number): string {
+  return hz >= 0 ? `+${hz}Hz` : `${hz}Hz`
+}