test(e2e): search palette + lesson detail + stats + legacy redirect

Add e2e/ux.spec.ts covering the ⌘K search palette, lesson detail page,
stats sections, and the legacy /admin → /lessons redirect.

Also fixes two issues uncovered by running the full suite:
- Skip auth rate limiter in e2e by running the backend with NODE_ENV=test
  (registration limit of 5 was tripping later tests).
- Render the card table for lesson owners even when the lesson has zero
  cards, so the first card can be added from the detail page.

packages/frontend/playwright.config.ts

@@ -6,7 +6,7 @@ export default defineConfig({
   webServer: [
     {
       command:
-        'rm -f packages/backend/data/e2e.db data/e2e.db && DB_PATH=./data/e2e.db SMTP_HOST=localhost SMTP_PORT=1025 APP_URL=http://localhost:5173 npm -w @flashcard/backend run db:migrate && DB_PATH=./data/e2e.db SMTP_HOST=localhost SMTP_PORT=1025 APP_URL=http://localhost:5173 npm -w @flashcard/backend run dev',
+        'rm -f packages/backend/data/e2e.db data/e2e.db && DB_PATH=./data/e2e.db SMTP_HOST=localhost SMTP_PORT=1025 APP_URL=http://localhost:5173 npm -w @flashcard/backend run db:migrate && NODE_ENV=test DB_PATH=./data/e2e.db SMTP_HOST=localhost SMTP_PORT=1025 APP_URL=http://localhost:5173 npm -w @flashcard/backend run dev',
       cwd: '../..',
       port: 3000,
       reuseExistingServer: false,