feat: switch Azure TTS voice to en-US-DavisNeural

Aria sounded soft for the English learning use case. Davis is
male, casual, with clearer articulation. Inline comment lists
alternative neural voices to try by swapping the constant.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/views/Editor/EnglishSpeaking/services/speechService.ts

@@ -1,6 +1,9 @@
 const KEY = import.meta.env.VITE_AZURE_SPEECH_KEY as string | undefined
 const REGION = import.meta.env.VITE_AZURE_SPEECH_REGION as string | undefined
-const VOICE = 'en-US-AriaNeural'
+// 美式男声,口齿清晰、随和。备选(都是 en-US-*Neural):
+//   GuyNeural / AndrewNeural / ChristopherNeural / TonyNeural(男)
+//   JennyNeural / EmmaNeural / AvaNeural(女,清晰度也高)
+const VOICE = 'en-US-DavisNeural'
 const FORMAT = 'audio-24khz-48kbitrate-mono-mp3'
 // 默认偏自然偏快;调慢改 -10%、调更快改 +25% 之类。
 const RATE = '+15%'