Add LM Model selector (0.6B/1.7B/4B) with auto-download

- Add lmModel parameter through full chain (types, API, routes, service)
- Add LM Model dropdown in Advanced Settings (defaults to 0.6B)
- Pass lmModel and lmBackend to format/enhance endpoint
- Update format_sample.py to accept --lm-model and --lm-backend args
- Auto-download model from HuggingFace if not present locally
- Persist model selection in localStorage
- Improve format route error logging with exit code and stdout/stderr

Fixes #9

server/src/services/acestep.ts

@@ -137,6 +137,7 @@ async function submitToApi(params: GenerationParams): Promise<{ taskId: string }
     use_cot_language: false, // Explicitly disable CoT features that require LLM
     use_cot_metas: false, // Explicitly disable CoT features that require LLM
     lm_backend: params.lmBackend || 'pt',
+    lm_model_path: params.lmModel || undefined,
   };
 
   if (params.duration && params.duration > 0) body.audio_duration = params.duration;
@@ -388,6 +389,7 @@ export interface GenerationParams {
   lmTopP?: number;
   lmNegativePrompt?: string;
   lmBackend?: 'pt' | 'vllm';
+  lmModel?: string;
 
   // Expert Parameters
   referenceAudioUrl?: string;
@@ -675,6 +677,7 @@ async function processGeneration(
     if (params.lmTopP !== undefined) args.push('--lm-top-p', String(params.lmTopP));
     if (params.lmNegativePrompt) args.push('--lm-negative-prompt', params.lmNegativePrompt);
     if (params.lmBackend) args.push('--lm-backend', params.lmBackend);
+    if (params.lmModel) args.push('--lm-model', params.lmModel);
     if (params.useCotMetas === false) args.push('--no-cot-metas');
     if (params.useCotCaption === false) args.push('--no-cot-caption');
     if (params.useCotLanguage === false) args.push('--no-cot-language');