feat: insert numbered image references from thumbnails

[miaomiaozii]

Summary

• clicking reference image N inserts a literal @N at the prompt caret or selection
• numbering is dynamic and follows the existing reference-image limit rather than being hardcoded to five images
• audio and video references do not consume image numbers
• drag and remove actions do not insert markers

Verification

• JavaScript syntax check passed
• interaction behavior was verified in the canvas UI