Print the message you asked to print

The Print in a message's own menu called window.print() bare, so it
printed the whole conversation -- every message on the page -- which is
what the toolbar's "Print conversation" is already for. Opening one
message's menu and asking to print it is not a request for the other
eleven.

It now marks the card it was opened from and the print stylesheet drops
the siblings for the duration. The subject heading stays: a printed
message with no subject on it is a page nobody can file. The "3 messages"
count beside it goes, since only one of them is on the paper.

The label is left as "Print" rather than made more specific -- it is
translated in all nine shipped catalogues, and the toolbar entry beside
it already says "conversation".

web/src/styles/app.css

@@ -1235,6 +1235,15 @@ select optgroup { background-color: var(--bg-elev); color: var(--fg); }
    */
   .message { break-inside: auto; border: 1px solid var(--border); }
   .message-head { break-inside: avoid; break-after: avoid; }
+  /*
+   * "Print" in a message's own menu marks that card and prints it alone;
+   * "Print conversation" in the thread toolbar sets no mark and prints them
+   * all. The subject heading stays either way -- a printed
+   * message with no subject on it is a page nobody can file -- but the
+   * "3 messages" count beside it goes, since only one of them is on the page.
+   */
+  .printing-one .message:not(.print-target) { display: none !important; }
+  .printing-one .thread-subject > .muted { display: none !important; }
   .print-only { display: block; }
   body { background: #fff; color: #000; }
 }