fix: fix placeholder icon name

[huhuanming]

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[what-the-diff]

PR Summary

• Change in Import Path in Icons.tsx
  The file Icons.tsx was adjusted. This change involves altering the import statement for PlaceholderOutline. Now, PlaceholderOutline is being imported from a different location, which could enhance software structure and maintainability.

• File Renaming
  The file originally named placeholder.tsx has been renamed to Placeholder.tsx. This alteration improves the consistency of our file naming conventions, which can yield easier navigation and understanding of the project structure for the team.