feat: polish CodeExample layout for narrow (mobile) viewports (Closes #941)

[waterWang]

Summary

Polishes the CodeExample component's layout for narrow (mobile) viewports per Issue #941. The mobile breakpoint has been expanded from 375px to 480px to cover a wider range of modern devices (iPhone 14, Pixel 7, Galaxy A series, etc.).

Changes

src/styles/code.css

• Expanded mobile breakpoint from max-width: 375px to max-width: 480px
• Merged the intermediate breakpoint (376–480px) into the single mobile breakpoint — all devices ≤480px now get the stacked column layout with 44px tap targets
• Updated all comments and documentation to reflect the new breakpoint

src/components/CodeExample.tsx

• Updated all JSDoc and inline comments to reference the new 480px breakpoint and Issue Polish CodeExample layout for narrow (mobile) viewports [b#087] #941

src/components/CodeExample.test.tsx

• Updated all Issue Polish CodeExample layout for narrow (mobile) viewports [b#007] #684 references to Issue Polish CodeExample layout for narrow (mobile) viewports [b#087] #941
• Added 4 new tests for the expanded breakpoint:
  • Header class contract for @media override
  • Copy button ghost-button class consistency
  • Pre element class for white-space switching
  • Tab strip fade mask class contract

Verification

NODE_ENV=development npx vitest run src/components/CodeExample.test.tsx
 ✓ 43 tests passed