feat(frontend): apply UI/UX design system - purple/green palette, gradient buttons, 3D flip, polished pages

Applied ui-ux-pro-max design system recommendations:
- Tailwind theme: study purple primary + correct green accent
- Inter + Plus Jakarta Sans typography
- Glassmorphic surfaces with soft shadows and mesh background
- Real 3D card flip with spring physics + answer feedback flash
- Gradient stat cards, progress bar, animated done screen with score ring
- Polished Layout, Dashboard, Admin, AdminLesson, CardTable, ImportDialog, PracticeSetup, Practice, PracticeDone
- E2E smoke updated for new accessible names

packages/frontend/playwright.config.ts

@@ -5,7 +5,7 @@ export default defineConfig({
   webServer: [
     {
       command:
-        'rm -f data/e2e.db && DB_PATH=./data/e2e.db npm -w @flashcard/backend run db:migrate && DB_PATH=./data/e2e.db npm -w @flashcard/backend run dev',
+        'rm -f packages/backend/data/e2e.db data/e2e.db && DB_PATH=./data/e2e.db npm -w @flashcard/backend run db:migrate && DB_PATH=./data/e2e.db npm -w @flashcard/backend run dev',
       cwd: '../..',
       port: 3000,
       reuseExistingServer: false,