feat: add buildFifeSuffix utility for high-res screenshot URLs

[icebear0828]

Summary

• Adds buildFifeSuffix() utility and FifeImageOptions type for appending FIFE sizing parameters to screenshot URLs
• Exports both from the main entry point so users can request higher-resolution images from getImage()

Fixes #152

Usage

import { buildFifeSuffix } from "@google/stitch-sdk";

const url = await screen.getImage();
const hiRes = url + buildFifeSuffix({ width: 780 });           // =w780
const both  = url + buildFifeSuffix({ width: 780, height: 1688 }); // =w780-h1688

E2E verification (real FIFE service)

Suffix	Dimensions	Size
(none)	187×512	47 KB
=w780	780×2132	345 KB
=w780-h1688	618×1688	316 KB

Test plan

• npm run test — 87 tests pass (includes 6 new fife.test.ts)
• npm run test:scripts — 57 tests pass
• npm run test:smoke — all smoke checks pass
• E2E: 3 rounds against live FIFE service, all resolve to valid images