test: add Playwright E2E tests for canvas mount, node navigation, transcription

Three spec files with beforeAll skip guards:
- Skip if VITE_TEST_TEACHER_EMAIL/PASSWORD not set
- Skip if dev server at 192.168.0.251:13000 unreachable
Tests exit 0 (3 skipped) when env is unconfigured.
Requires: live dev server + real teacher credentials to run live.

playwright.config.ts

@@ -0,0 +1,18 @@
+import { defineConfig, devices } from '@playwright/test';
+
+export default defineConfig({
+  testDir: './src/__tests__/e2e',
+  testMatch: '**/*.spec.js',
+  timeout: 30000,
+  expect: { timeout: 5000 },
+  fullyParallel: false,
+  retries: 1,
+  use: {
+    baseURL: process.env.PLAYWRIGHT_BASE_URL || 'http://192.168.0.251:13000',
+    trace: 'on-first-retry',
+  },
+  projects: [
+    { name: 'chromium', use: { ...devices['Desktop Chrome'] } },
+  ],
+  webServer: undefined,
+});