Complete workflow liberalization following PR #24627:
1. Remove Start node deletion restriction by removing isUndeletable property
2. Fix draft sync blocking when no Start node exists
3. Restore isWorkflowDataLoaded protection to prevent race conditions
4. Ensure all entry nodes (Start + 3 trigger types) have equal deletion rights
This allows workflows with only trigger nodes and fixes the issue where
added nodes would disappear after page refresh due to sync API blocking.
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Co-Authored-By: Claude <noreply@anthropic.com>
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