Improve translation prompt to avoid outputting source language

Made-with: Cursor

Author: Hongwei-MB

src/LiveLingo.Core/Engines/LlamaTranslationEngine.cs

@@ -49,8 +49,8 @@ public async Task<string> TranslateAsync(
         var endpoint = await _host.GetOrStartServerAsync(ct);
         var url = $"{endpoint}/completion";
 
-        var systemPrompt = $"You are a professional translator. Translate the user's text from {srcName} to {tgtName}. Output ONLY the translated text, nothing else. Do not output any thought process or explanation. Do not use <think> tags.";
-        var prompt = $"<|im_start|>system\n{systemPrompt}<|im_end|>\n<|im_start|>user\n{text}<|im_end|>\n<|im_start|>assistant\n";
+        var systemPrompt = $"You are an expert translation engine. Translate the text from {srcName} to {tgtName}.\nRules:\n1. Output ONLY the translated text in {tgtName}.\n2. Do NOT repeat or output the original {srcName} text.\n3. Do NOT add any explanations, notes, or conversational text.\n4. Do NOT use <think> tags or output any thought process.";
+        var prompt = $"<|im_start|>system\n{systemPrompt}<|im_end|>\n<|im_start|>user\nTranslate this to {tgtName}:\n\n{text}<|im_end|>\n<|im_start|>assistant\n";
 
         _logger.LogDebug("Translation prompt for {Src}→{Tgt}: {Prompt}", sourceLanguage, targetLanguage, prompt);