fix: deduplicate dashboard notification events

[CleanDev-Fix]

Summary

Fixes duplicate notification rendering after rapid refreshes by deduplicating dashboard events before they reach the render layer.

Changes

• Added event deduplication by stable eventId in the dashboard event store.
• Applied deduplication to both setEvents and appendEvents so refresh payloads and appended updates cannot create duplicate rendered entries.
• Added regression tests covering duplicate render input and repeated append data.

Testing/Verification

• npm ci --ignore-scripts from dashboard/ passed. Plain npm ci hits a Windows-incompatible transitive postinstall command (yarn setup || true) unrelated to this change.
• npm test -- --runTestsByPath src/store/eventStore.test.tsx --runInBand passed.
• npm test -- --runInBand passed: 5 suites / 22 tests.
• npm run build passed. Vite reported pre-existing duplicate devDependency key warnings in dashboard/package.json.
• node ./node_modules/eslint/bin/eslint.js "src/**/*.{ts,tsx}" --max-warnings=0 passed. The package npm run lint script uses Unix single-quoted globs, which fail on Windows.

Related issue: closes #134