Fix template theme hydration

[zahlekhan]

Summary

• use useSyncExternalStore with a deterministic server snapshot in the OpenUI Cloud template and example
• add the same SSR-safe system-theme provider to the self-hosted CLI template
• pass the resolved theme mode into the self-hosted AgentInterface
• align the OSS openui-chat example with the template behavior

Root cause

The theme hook used a lazy useState initializer that returned light during server rendering but read prefers-color-scheme during the browser's first render. For users in dark mode, React hydrated light server markup with dark client properties, including a different logo URL, and reported an unrecoverable attribute mismatch.

useSyncExternalStore now supplies light as React's server and hydration snapshot, then switches to the live media-query snapshot after hydration and continues subscribing to system-theme changes.

Impact

Cloud and self-hosted apps generated by the CLI now hydrate deterministically while retaining responsive system light/dark mode behavior. The checked-in Cloud and OSS examples use the same implementation.

Validation

• Cloud template: npm test (3 passed)
• Cloud template: npm run typecheck
• Cloud template: npm run lint (passes with one pre-existing no-img-element warning)
• Cloud template: npm run build
• Cloud template: dark-mode browser smoke test with no hydration errors
• Self-hosted template: npm run lint
• Self-hosted template: npx tsc --noEmit
• Self-hosted template: OPENAI_API_KEY=build-validation-placeholder npm run build