feat(ui): image paste from clipboard in editor #425

Summary

Users should be able to paste images from the clipboard directly into the editor (both visual/BlockNote mode and source/CodeMirror mode). The pasted image gets uploaded as an asset and inserted as a markdown image reference. This is a standard feature in Notion, Confluence, Obsidian, GitHub, and every modern editor.

Proposed behavior

1. Paste (Ctrl+V / Cmd+V) with image data on clipboard → upload to POST /api/kiwi/assets → insert ![](assets/<filename>) at cursor position.
2. Drag-and-drop — same flow for images dragged into the editor area.
3. File naming: paste-<timestamp>.<ext> (e.g., paste-20260623-143022.png).
4. Show a loading placeholder while uploading (e.g., ![Uploading...]()), then replace with the real URL on success.
5. Support formats: PNG, JPEG, GIF, WebP.
6. Error handling: if upload fails, show a toast notification and remove the placeholder.

Implementation notes

• Visual mode (BlockNote): BlockNote already handles some paste events. Hook into the editor.onPaste or use a ProseMirror plugin to intercept clipboardData.items with type.startsWith('image/').
• Source mode (CodeMirror): Use CodeMirror's EditorView.domEventHandlers to handle paste and drop events.
• Upload endpoint: POST /api/kiwi/assets (already exists, accepts multipart form data).
• Prioritize shadcn components. Use shadcn Sonner / toast for upload progress and error notifications — install via npx shadcn@latest add sonner if not present. Alternatively use the existing toast mechanism if one exists in the codebase.
• Max file size: respect the [assets] max_file_size config (default 10MB).
• For drag-and-drop, add a visual drop zone overlay using a div with dashed border that appears on dragover.

Acceptance criteria

• Pasting an image from clipboard uploads and inserts markdown image
• Works in both visual (BlockNote) and source (CodeMirror) modes
• Drag-and-drop images into editor works
• Loading placeholder shown during upload
• Error toast on upload failure
• Respects max file size config
• Unit/integration tests for paste handler