fix(app): use native macOS screen fonts

[Limdongcheng]

Summary

• use the native macOS system UI font for proportional screen text
• add Hiragino Sans GB as the Simplified Chinese fallback while preserving Phosphor icon priority
• keep monospace text, non-macOS behavior, and SVG/EMF exports unchanged without bundling Apple fonts

Validation

• cargo pr-check
• cargo test -p plotx --no-default-features
• macOS screenshot harness in light and dark themes at 720, 900, and 1440 widths
• manual inspection for clipping, overlap, tofu glyphs, icon regressions, and canvas preview rendering

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[Limdongcheng]

I have read the CLA Document and I hereby sign the CLA