Add petstore SPA UI with Playwright e2e tests

[Copilot]

Adds a vanilla JS single-page application for the petstore covering all APIs, plus a full Playwright test suite that runs against the real Counterfact backend.

UI (ui/index.html)

Single-file SPA with three tabs, no build step required:

• Pets — browse by status, search by tags, add/edit (modal)/delete
• Store — live inventory, place/lookup/delete orders
• Users — list, add, edit, lookup, delete

API base URL defaults to http://localhost:3100; overridable via ?api=<url> query param (used by tests to isolate on port 3101).

Tests (test/petstore.ui.test.ts)

22 Playwright tests across all three tabs, started with npm run test:ui. Config in playwright.config.ts spins up both the Counterfact backend and a serve static file server automatically. Shared TEST_API_URL extracted to test/constants.ts to keep config and tests in sync.

Route handler fixes

Counterfact passes URL path parameters as strings at runtime. All handlers that look up entities by numeric ID now coerce explicitly:

// Before — strict equality against number always false for string "1"
const pet = $.context.getPetById($.path.petId);

// After
const pet = $.context.getPetById(Number($.path.petId));

Affected: pet/{petId}.ts, pet/{petId}/uploadImage.ts, store/order/{orderId}.ts.

package.json scripts

Script	Purpose
test	Node built-in runner + tsx loader for unit tests
test:ui	Playwright e2e suite
start:test	Counterfact on port 3101 (test isolation)