[SandObserver]

Summary

• Cut rationale, history and narrative from docs/design-system.md; every measured value and table is kept. 2196 words to 1536.
• Update the touch-slider paragraph to the per-axis radius the control uses since Fix: iOS screen fill, app icons, and touch slider ends #90.
• Reword comments, docs and test descriptions so platform behaviour is described in neutral terms rather than by vendor name.
• Rename two device strings in a nowplaying test fixture; no code matches them.
• Add a short "Platform identifiers" note to docs/frontend.md recording which vendor names are identifiers rather than references.
• Restamp the cache-busting versions the edited modules cascade through.

Identifiers are deliberately unchanged, because renaming them breaks the app: the -apple-system / BlinkMacSystemFont font-family keywords, the apple-touch-icon and apple-mobile-web-app-* meta names read at add-to-homescreen time, the stored showLabels.ios config key, user-agent patterns, the Playwright device name and __MACOSX/ in .gitignore. CHANGELOG.md is untouched as a historical record.

Testing
All green: 1184 backend tests, 801 frontend, lint, format, both typechecks, cache-busting check and the Docker smoke build on the homelab. No behaviour changes and no tests added; the edits are comments, docs, and two fixture strings.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.