Put a code on the back of a card, a picture on the front, and the page in the app

The back side landed last commit carrying text by default, and a back laid out
as text-only renders no code box at all. So a two-sided run came out with a
blank reverse, which reads as the code having failed rather than as a setting
doing exactly what it said. The back now defaults to carrying a code, takes its
own content per row, and chooses its own symbology - a QR on the front that
opens the record, a CODE128 on the back that the bench scanner already reads.
Text-only is refused while the back carries a code, and the contradiction is
reconciled on load, so a saved setting from the broken combination repairs
itself rather than printing another blank box of cards.

A picture can go on a label: above the code, beside it, or behind everything as
a watermark, on either side or both. The file is read in the browser and never
sent anywhere, which is what makes it usable at an air-gapped site, and it is
deliberately not saved between visits - a data URL runs to megabytes and would
take every sizing value with it when the quota gave out. The watermark says
plainly that it costs the scanner contrast.

The generator was a full-screen route with no navigation and a narrow column of
controls against the left edge. That shape exists so a print cannot put the
sidebar on label stock, which is a real hazard and the reason core's label pages
are built that way - but it is a hazard the print stylesheet can answer, and
style.css now hides the shell at print time. A tool someone spends ten minutes
in gets the navigation and the full width; the stock still gets nothing but
labels, confirmed against a print PDF rather than a preview.

Two smaller things found while checking the above. A test print paired each page
with a rendered code by loop position, while the codes are keyed to position in
the whole run, so a two-sided test card could carry the wrong code; the index
now travels with the page. And the back's dotted preview outline outspecified
the print reset, drawing a dashed rule down the edge of every second card.

frontend/src/assets/style.css

@@ -1763,3 +1763,31 @@ td.actions .btn + .btn {
    independently. */
 .clickable-row { cursor: pointer; }
 .clickable-row:hover td { background: var(--bg); }
+
+/* --- Printing from inside the app shell -------------------------------------
+ *
+ * Core's label pages are full-screen routes precisely so the shell cannot reach
+ * the stock. A tool someone works in for ten minutes wants the navigation
+ * though, so the shell is hidden here instead: the sidebar goes, the content
+ * column gives back the margin it was holding for it, and the notification
+ * banner - which is chrome, not content - goes with them.
+ *
+ * Anything the page itself must keep off the paper marks itself .no-print. */
+@media print {
+  .sidebar,
+  .notification-banner,
+  .no-print {
+    display: none !important;
+  }
+
+  .app-layout {
+    display: block !important;
+    min-height: 0 !important;
+  }
+
+  .main-content {
+    margin-left: 0 !important;
+    padding: 0 !important;
+    overflow: visible !important;
+  }
+}