Add raw AI response to parse result for debugging

- Backend returns raw AI text alongside parsed fields
- Drawer shows expandable "Raw AI response" section when result has empty fields
- Scan Receipt shows raw response inline if no fields were extracted
- Helps diagnose model output issues without needing server logs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/settings.ts

@@ -21,6 +21,7 @@ export interface ParsedReceipt {
   date: string | null;
   description: string | null;
   category: string | null;
+  raw: string | null;
 }
 
 export async function getAiSettings(): Promise<AiSettings> {