test: Playwright E2E with real backend + fake TTS engine

e2e/ drives the web build (vite preview) against a per-worker uvicorn:
each Playwright worker boots its own backend on its own port with a
temp cwd, so SQLite and audio files are fully isolated and parallel-
safe. The page fixture seeds the persisted voicebox-server store with
the worker's URL before any script runs.

VOICEBOX_FAKE_TTS=1 short-circuits get_tts_backend_for_engine to a
backend that synthesizes a sine tone sized to the text — the real task
queue, SSE progress, history rows, and audio serving all run, only
inference is fake. backend/requirements-ci.txt is the slim dependency
set validated to boot the app on a CPU-only runner.

Specs: startup, settings layout, seeded profile in /voices, and
generate-to-completed-audio through the full pipeline.

e2e/specs/startup.spec.ts

@@ -0,0 +1,16 @@
+import { expect, test } from '../fixtures';
+
+test('app boots against the backend and shows the main editor', async ({ page }) => {
+  await page.goto('/');
+
+  await expect(page.getByRole('heading', { name: 'Voicebox' })).toBeVisible();
+  await expect(page.getByRole('button', { name: 'Create Voice' }).first()).toBeVisible();
+});
+
+test('settings layout renders its pages', async ({ page }) => {
+  await page.goto('/settings/about');
+
+  await expect(page.getByText('General', { exact: true })).toBeVisible();
+  await expect(page.getByText('Changelog', { exact: true })).toBeVisible();
+  await expect(page.getByText('About', { exact: true })).toBeVisible();
+});