First visit ends in a sheet, not in an empty box

A fresh print-only install showed "Pick from the library" as the second
way in — on an instance that has no library, because the library belongs
to the AI module. The button opened an empty picker. It is gone when the
module is off (`hasLibrary`), together with the two smaller "from the
library" links.

In its place: "Try it with a sample photo". It fetches
public/samples/lake-at-dusk.jpg — an original illustration drawn for this,
no third-party material — and sends it through the normal upload, so
everything after it is what happens with a real picture.

Checked in the browser, English and German, on a print-only instance:
no library button, sample loads, the print button enables, the PDF
measures 210.00 x 297.00 mm, no failed request.

public/samples/README.md

@@ -0,0 +1,9 @@
+# Sample image
+
+`lake-at-dusk.jpg` is an original illustration drawn for Klarbild as vector
+shapes and rendered to JPEG. It contains no third-party material and is
+covered by the repository's licence like everything else here.
+
+It exists so that the first visit to a fresh instance can end in a printed
+sheet: the "Try it with a sample photo" button in the print studio uploads
+it through the normal upload path.