Refactor desktop typography controls / 重构桌面端字体控制

[SivanCola]

Summary

• Refactor desktop typography into platform-aware UI/content/monospace font tokens.
• Add interface text-size scaling up to an extra accessibility step with status-bar/layout overflow safeguards.
• Add UI and monospace font controls, custom font stacks, and platform/local-font availability filtering.
• Add source-level regression contracts for font availability and typography overflow behavior.

Verification

• CSS syntax check passed.
• z-index token check passed.
• git diff --check passed.

Full local tsx/tsc verification was not run in this worktree because the frontend dependencies and package-manager binaries were not installed locally.

[SivanCola]

@codex review

[chatgpt-codex-connector]

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[SivanCola]

approve