Follow device appearance or keep a Light / Black preference

[zxxma]

Summary

Device appearance was only read at startup, and the UI offered no way to return to System after a manual choice. Add System / Light / Black in the desktop top bar, mobile More menu and Settings, with ongoing OS synchronization, cross-tab updates and safe storage fallback. Preserve existing dark/light values and the resolved CSS-theme contract.

This is the theme foundation for the professional-mainnet pilot in #1186. Validators palette/layout work remains a separate default-off PR (#1195). No network, contract or authentication flags change. The branch incorporates mainline d49896a and preserves the landed validator accessibility and protobuf changes.

Test plan

• Final branch d3028e8 passes full GitHub CI: Node 20/22 frontend, full Chromium E2E, mobile guardrails, backend, Docker, workspace and protobuf checks.
• Initial local full frontend suite: 503 files / 5,002 tests passed, one existing skip. Refreshed local production build/lint passed; the duplicate local unit rerun was stopped once equivalent exact-branch CI passed.
• Dedicated Chromium theme behavior test passed: OS following, manual override/reload and returning to System.
• All 20 local iPhone shell/validator guardrails passed after removing the redundant mobile top-bar picker, retaining More/Settings. Includes the new 320px header and More-menu case.
• The desktop network-picker test now targets its accessible name so adding a separate theme picker does not make the locator ambiguous; all three affected local top-bar tests passed, followed by green full desktop CI.
• Full backend build/race suite and Buf lint passed before the final frontend-only mainline refresh; final remote CI also passes these checks.

Draft for review; no automatic merge or deployment.

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Lighthouse	1 paths audited Performance: 73 (no change from production) Accessibility: 97 (no change from production) Best Practices: 83 (no change from production) SEO: 90 (no change from production) PWA: 100 (no change from production) View the detailed breakdown and full score reports

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[github-actions]

The latest Buf updates on your PR. Results from workflow CI / proto (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Sep 14, 2026, 8:07 PM

[zxxma]

Closing as superseded by #1200 (professional frontend design rollout), which shipped the theme preference work on main; this branch now conflicts in 9 files. The branch is kept for reference.