feat: redesign day picker with expandable calendar, status markers & Playwright E2E testing

- Replace modal calendar with inline expandable calendar widget integrated into the day picker.
- Add status indicator dots (attended/absent/pending) to month calendar grid matching day picker.
- Disable and gray out weekend days in calendar when weekend schedule is turned off.
- Redesign extend bar with integrated handle that aligns with the palette.
- Remove start-to-end timing on Today screen's class time rail, showing only start time.
- Implement Playwright E2E and visual testing suite with multi-screen capture scripts.
- Add cascade DELETE endpoints for subjects and versioned recurring timetable entries.

playwright.config.ts

@@ -0,0 +1,28 @@
+import { defineConfig, devices } from '@playwright/test';
+
+export default defineConfig({
+  testDir: './e2e',
+  timeout: 30000,
+  use: {
+    baseURL: 'http://127.0.0.1:3000',
+    trace: 'on-first-retry',
+  },
+  projects: [
+    {
+      name: 'Mobile Chrome',
+      use: { ...devices['Pixel 5'] },
+    },
+  ],
+  webServer: [
+    {
+      command: 'node backend/server.mjs',
+      port: 4000,
+      reuseExistingServer: !process.env.CI,
+    },
+    {
+      command: 'node scripts/serve-dist.mjs',
+      port: 3000,
+      reuseExistingServer: !process.env.CI,
+    },
+  ],
+});