feat: adds PDF import with conversion to JSON 1.0

- Accept only PDF and JSON files in import modal and API
- Convert PDF statements to JSON 1.0 via LLM (OpenAI-compatible)
- Use multipart/form-data for file upload (multer, 15 MB limit)
- Add LLM_API_KEY and LLM_API_BASE_URL for configurable LLM endpoint
- Update ImportModal to validate type and send FormData
- Add postFormData to API client for file upload

frontend/src/api/import.ts

@@ -2,7 +2,12 @@ import type { ImportStatementResponse } from '@family-budget/shared';
 import { api } from './client';
 
 export async function importStatement(
-  file: unknown,
+  file: File,
 ): Promise<ImportStatementResponse> {
-  return api.post<ImportStatementResponse>('/api/import/statement', file);
+  const formData = new FormData();
+  formData.append('file', file);
+  return api.postFormData<ImportStatementResponse>(
+    '/api/import/statement',
+    formData,
+  );
 }