fix(voice): remove hard-coded Chinese language from Gemini backend

buildGeminiLiveConfig was appending VOICE_CHINESE_LANGUAGE_BLOCK which
forced Gemini to always respond in Mandarin regardless of user locale.
Gemini now uses the neutral base prompt and responds in the language the
user speaks to it, consistent with the ElevenLabs behaviour.

Author: heavygee

shared/src/voice.ts

@@ -344,7 +344,7 @@ export function buildGeminiLiveFunctionDeclarations(): GeminiLiveFunctionDeclara
 export function buildGeminiLiveConfig(): GeminiLiveConfig {
     return {
         model: GEMINI_LIVE_MODEL,
-        systemInstruction: VOICE_SYSTEM_PROMPT + VOICE_CHINESE_LANGUAGE_BLOCK,
+        systemInstruction: VOICE_SYSTEM_PROMPT,
         tools: [
             {
                 functionDeclarations: buildGeminiLiveFunctionDeclarations()