Fix Appearance color scheme overrides

[huytdps13400]

Summary

• implement Appearance.setColorScheme with light and dark overrides
• restore the system preference with auto, deprecated unspecified, or null
• notify listeners only when the effective color scheme changes, while suppressing system changes during an override
• document the API and add regression coverage for overrides, resets, subscriptions, and listener removal during dispatch

Fixes #2703.

Verification

• npm run test — 47 DOM suites / 715 tests passed; 3 Node suites / 6 tests passed
• npm run build -w react-native-web — Flow, ESM, and CommonJS builds passed
• Chromium runtime check in the Next.js examples app — light → dark → auto updated the subscribed value correctly; no console warnings or errors
• git diff --check

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