Paste and drag-and-drop of image files upload via the #27 API and insert
a real image node only once the upload succeeds; the in-flight state is
a ProseMirror decoration, not a document node, so a failed upload cannot
leave anything broken behind (it shows a transient inline error instead).
The toolbar's image button opens a native file picker into the same
upload path. Selecting an image reveals inline alt-text and width-preset
(small/medium/full) controls. Also fixes the image node's parseDOM,
which had no getAttrs and would drop the required fileId attribute on
internal copy/paste.
Closes#28
