[Studio] Harden cluster config and store loading

[Aias00]

Summary

• validate cluster flushDiskType values before applying config updates
• load clusters in the frontend cluster store through the shared cluster service
• keep cluster store loading state reset after load failures
• consolidate related cluster config/store PRs into one review unit

Supersedes

#600 #612

Tests

• JAVA_HOME=$(/usr/libexec/java_home -v 21) mvn -q -Dtest=ClusterServiceTest test
• npm ci --ignore-scripts --no-audit --no-fund && npm test -- --run src/stores/clusterStore.test.ts
• git diff --check

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[Aias00]

Superseded by #645. Folded into the consolidated cluster repository/config state PR because cluster identity ordering and config/store loading are the same cluster state stability surface.