test(e2e): run full suite at mobile viewport too (Pixel 5)

Adds a second Playwright project so every calculator test runs at both
Desktop Chrome and Pixel 5 (~375 px). Catches mobile layout regressions
automatically. 106 tests passing (53 × 2 viewports).

e2e/playwright.config.js

@@ -20,5 +20,7 @@ module.exports = defineConfig({
   },
   projects: [
     { name: 'chromium', use: { ...devices['Desktop Chrome'] } },
+    // Mobile pass — catches layout regressions at ~375 px (iPhone SE)
+    { name: 'mobile-chrome', use: { ...devices['Pixel 5'] } },
   ],
 });