Add API client module (axios/fetch wrapper with base URL and error handling) #4

Task

Before wiring up the frontend to the backend, create a centralized API client so all fetch calls are consistent and maintainable.

Suggested: frontend/src/api/client.ts

const BASE = import.meta.env.VITE_API_URL ?? 'http://localhost:8000/api';

export async function uploadDataset(file: File) { ... }
export async function listDatasets() { ... }
export async function runAnalysis(filename: string) { ... }
export async function runAgentAnalysis(filename: string, task?: string) { ... }

Acceptance Criteria

• Single module for all API calls
• VITE_API_URL env var used for base URL (so it works in both local dev and deployed)
• All functions return typed responses matching backend Pydantic schemas
• Errors throw with a consistent { message, status } shape
• Add frontend/.env.example with VITE_API_URL=http://localhost:8000/api