fix: prevent image block bubble menu from disappearing on hover

[j4rviscmd]

Summary

Fixes #40 — The image block's formatting toolbar (bubble menu) disappears before the cursor can reach it because the hoverable area is limited to the image element itself when no caption is present.

Changes

1. Auto-set caption on new image uploads (imageUpload.ts)

• Changed uploadImage to return an object { props: { url, name, caption } } instead of a plain URL string
• BlockNote uses the returned object directly in editor.updateBlock(), setting the caption to the filename
• The rendered caption element expands the hoverable area, keeping the toolbar accessible

2. CSS hover area expansion for captionless images (index.css)

• Added a ::after pseudo-element (20px height) on [data-content-type="image"] > .bn-file-block-content-wrapper
• Only applies when no caption is present (:not(:has(.bn-file-caption))) and a preview is shown (:has(.bn-visual-media-wrapper))
• Provides an invisible hover zone beneath the image so the toolbar remains reachable

3. Cleanup

• Removed abandoned custom ImageBlock approach (imageBlock.tsx, related imports in Editor.tsx and index.ts)
• The custom block approach was abandoned because mixing defaultBlockSpecs (vanilla JS) with createReactBlockSpec (React) breaks resize handles

Testing

• New image upload: caption auto-set to filename, toolbar accessible
• Existing captionless images: toolbar reachable via CSS hover zone
• Resize handles work correctly
• cargo build passes
• npm run build passes