feat: improve responsive layout

[wendyyuchensun]

Summary

• Raise the side-by-side breakpoint from 768px (md) to 1024px (lg) so the editor and preview each have enough horizontal room before sharing a row. Below 1024px, the preview stacks full-width beneath the editor.
• Enforce a 768px minimum content width across header and main. Below a 768px viewport, the page scrolls horizontally as one unit instead of compressing the layout.
• Tighten header and toolbar button widths at smaller breakpoints so all controls fit on one row without breaking Chinese labels mid-character.

Demo

Before

Screen.Recording.2026-05-18.at.19.23.13.mov

After

Screen.Recording.2026-05-18.at.19.24.19.mov

[wendyyuchensun]

@jamliaoo 十分感謝！