[refactor] Extract dialog composables and add showSmallDialog helper

[viva-jinyi]

Summary

• Added showSmallDialog helper function to dialogService for reusable small dialog styling
• Extracted showLoadWorkflowWarning and showNodeConflictDialog into independent composables
  • useMissingNodesDialog
  • useNodeConflictDialog
• This is foundational work for also displaying MissingModels warning in a small modal

Changes

• dialogService.ts: Added showSmallDialog helper, removed migrated functions
• useMissingNodesDialog.ts: New composable for Missing Nodes dialog
• useNodeConflictDialog.ts: New composable for Node Conflict dialog
• Migrated existing usages to the new composable pattern

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