[codex] Fix React hydration mismatch on Dashboard loading state

[barry01-hash]

Summary

• Replace the dashboard's timeout-based hydration gate with a server/client snapshot gate using useSyncExternalStore.
• Split the dashboard into a loading shell wrapper and a ready content component so the initial markup stays deterministic.
• Add a regression test that checks the server-rendered loading shell and verifies hydration completes without console mismatches.

Validation

• npm test -- --run tests/components/DashboardHydration.test.tsx
• npx eslint app/dashboard/page.tsx tests/components/DashboardHydration.test.tsx

[github-actions]

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