feat: add chunk crossfade slider (0ms = hard cut)

Persisted setting (default 50ms) controls how audio chunks are blended
together.  Set to 0 for a clean hard cut with no overlap.

app/src/lib/api/types.ts

@@ -37,6 +37,7 @@ export interface GenerationRequest {
   engine?: 'qwen' | 'luxtts' | 'chatterbox' | 'chatterbox_turbo';
   instruct?: string;
   max_chunk_chars?: number;
+  crossfade_ms?: number;
 }
 
 export interface GenerationResponse {