Issue #3534314: Add search functionality to /media/add/unsplash

• Added autocomplete search to the Unsplash ID field on media add form
• Added autocomplete search to the Unsplash ID field on media edit form
• Added placeholder text to guide users
• Reuses existing autocomplete route from media library implementation

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

Closes #3534314