refactor(editor): remove separate caption concept and unify image name

[j4rviscmd]

Summary

• Remove CaptionButton and CaptionDialog components (caption editing UI)
• Unify image block naming: users only interact with name; caption is auto-synced to name
• RenameDialog now saves both name and caption props simultaneously
• getImageNameFallback replaces getImageCaptionFallback, syncing caption = name during backfill

Context

Users were confused by having separate name and caption concepts for image blocks.
Since rename value can serve as caption, the caption editing UI is redundant.
This simplifies the mental model: only name matters, and caption stays synced behind the scenes.