Paste an image from the clipboard

[johnkattenhorn]

Closes #64.

Ctrl+V pastes an image. It goes to an images folder beside the document, and
the document gets a relative reference to it:

![](images/pasted-20260917-142530.png)

Relative and under the document's own folder, so the document and its pictures
move, sync and open together — and so anything else that reads Markdown resolves
them the same way.

What it handles

• Clipboard pixels — a screenshot, or Copy Image from a browser. Written as
  PNG.
• A file on the clipboard — copied from a file manager. Copied rather than
  re-encoded, so whatever encoding and compression it already had survives.

Both run after the existing link paste, so a selection with a URL behind it
still becomes a link rather than a copy of the image.

What it refuses

An untitled document. It has no folder of its own, so an image put anywhere
would not survive being saved somewhere else. The status line says
Save the document first, then paste the image. rather than guessing a
location.

Naming

The clock names each image, so pastes stay in the order they were made and sort
that way in the folder. A counter goes behind it for two pastes inside the same
second.

Tests

Two, both in tst_omawrite.cpp:

• writesAPastedImageBesideTheDocument — the reference resolves beside the
  document, the file is on disk at the right size, and a second paste in the
  same second takes a name of its own rather than writing over the first.
• refusesToPasteAnImageIntoAnUntitledDocument — nothing written, and the
  status says why.

./bin/test — 14 passing.

Note

No new dependency, no UI, no new shortcut: Ctrl+V already had a paste path and
this is one more thing it recognises. Written against master at 8f98892.