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.

scripts/test-onboarding.mjs

@@ -0,0 +1,34 @@
+import { chromium } from 'playwright';
+
+async function testOnboarding() {
+  const browser = await chromium.launch({ headless: true });
+  const context = await browser.newContext({
+    viewport: { width: 390, height: 844 },
+    deviceScaleFactor: 2,
+    isMobile: true,
+  });
+  const page = await context.newPage();
+  
+  await page.goto('http://127.0.0.1:3000/onboarding');
+  await page.waitForTimeout(1000);
+  
+  // Fill step 1
+  await page.fill('input[placeholder="e.g. Sam Taylor"]', 'Alex Morgan');
+  await page.fill('input[placeholder="e.g. Northbridge University"]', 'Stanford University');
+  await page.click('text=Continue');
+  await page.waitForTimeout(1000);
+  
+  await page.screenshot({ path: './screenshots/08-onboarding-step2.png' });
+  
+  // Add a subject
+  await page.fill('input[placeholder="e.g. Data Structures"]', 'Algorithms');
+  await page.fill('input[placeholder="e.g. CS201"]', 'CS161');
+  await page.click('text=Add subject');
+  await page.waitForTimeout(500);
+
+  await page.screenshot({ path: './screenshots/09-onboarding-with-subject.png' });
+
+  await browser.close();
+}
+
+testOnboarding().catch(console.error);