test: replace ad-hoc tests with a layered suite and CI workflow

Splits testing into five layers so a failure points at the responsible one:

- tests/unit    isolated utility, repository and service tests
- tests/data    validates the tracked Pokémon, game, region and dex files
- tests/integration  schema, views, constraints, RLS and repositories
- tests/bdd     executable Gherkin for user-visible behaviour
- tests/build   service worker and manifest artifacts per build variant

Replaces the two Playwright specs in client-test/ and the two Vitest files in
test/. Adds a GitHub Actions workflow running the layers as separate jobs, a
mock OAuth provider server so the Drive and Dropbox scenarios never touch real
accounts, and a wrapper that reads the local Supabase keys from
`supabase status` rather than hard-coding them.

Extracts the pure formatting helpers out of PokedexExportService so they can be
unit tested, and makes the provider endpoints configurable so the mock server
can stand in for Google and Dropbox.

tests/bdd/steps/pwa.steps.ts

@@ -0,0 +1,55 @@
+import { createBdd } from 'playwright-bdd';
+import type { Page } from '@playwright/test';
+import { test, expect } from '../fixtures';
+
+const { Given, When, Then } = createBdd(test);
+
+async function waitForServiceWorker(page: Page) {
+	return page.evaluate(async () => {
+		if (!('serviceWorker' in navigator)) throw new Error('Service workers are not supported');
+		const registration = await Promise.race([
+			navigator.serviceWorker.ready,
+			new Promise<never>((_, reject) =>
+				setTimeout(() => reject(new Error('Service worker registration timed out')), 15_000)
+			)
+		]);
+		return registration.active?.scriptURL ?? null;
+	});
+}
+
+When('I open the built application', async ({ page }) => {
+	await page.goto('/');
+	await waitForServiceWorker(page);
+});
+
+Given('I have opened the built application online', async ({ page }) => {
+	await page.context().setOffline(false);
+	await page.goto('/');
+	await waitForServiceWorker(page);
+	await page.reload();
+});
+
+When('I go offline and revisit the home page with a trailing slash', async ({ page }) => {
+	await page.context().setOffline(true);
+	await page.goto('/', { waitUntil: 'domcontentloaded' });
+});
+
+When('I go offline and then return online', async ({ page }) => {
+	await page.context().setOffline(true);
+	await page.reload({ waitUntil: 'domcontentloaded' });
+	await page.context().setOffline(false);
+	await page.reload({ waitUntil: 'domcontentloaded' });
+});
+
+Then('a service worker controls the page', async ({ page }) => {
+	expect(await waitForServiceWorker(page)).toMatch(/\/(?:sw|prompt-sw)\.js$/);
+});
+
+Then('the application cache is present', async ({ page }) => {
+	const cacheNames = await page.evaluate(() => caches.keys());
+	expect(cacheNames.length).toBeGreaterThan(0);
+});
+
+Then('the application remains available', async ({ page }) => {
+	await expect(page.getByRole('heading', { name: /Start Your Pokédex Journey/ })).toBeVisible();
+});