feat(export): add configurable document fonts

[murongg]

Summary

• add a searchable installed-font selector under Settings > Export for HTML and PDF
• persist the selected family and safely inject it into standalone export styles
• reuse the editor font selector and add localized labels for every supported language

Validation

• pnpm --filter @markra/app test — 128 files and 1455 tests passed
• pnpm --filter @markra/shared test — 7 files and 62 tests passed
• pnpm --filter @markra/app typecheck:test passed
• pnpm build passed for the full workspace, including desktop and web builds
• Not run: manual Windows UI QA

Risk

• exported HTML references the selected installed font by name; it does not embed font files, so other machines without that font use the existing serif fallback stack

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview, Comment	Jul 28, 2026 3:30pm