fix: a desktop Plasma makes after the layout has run gets the classroom's wallpaper

Tiny Pebbles came through the 0.2.2 upgrade on Plasma's own purple
wallpaper: its desktop was containment 10, made after the layout had run
(most likely the television arriving after a night-time reboot), with no
wallpaper configured. The look and feel's fallback could not catch it,
because it named a file and Plasma only accepts a wallpaper package there.

The fallback is now the EkklesiaClassroom package, whose one image links to
the classroom's picture. classroom-desktop-wallpaper, run before every
plasmashell start, writes the picture into every desktop containment the
file already has: found by plugin, never by number; panels and everything
else byte for byte; a file that is already right not written at all.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

systemd/user/plasma-plasmashell.service.d/ekklesia-classroom.conf

@@ -16,3 +16,9 @@ StartLimitIntervalSec=0
 [Service]
 Restart=always
 RestartSec=2
+# Every desktop the configuration already has gets the classroom's
+# wallpaper written into it, in the one moment the shell is not running to
+# own the file — at a login, and at every restart after a crash. See
+# classroom-desktop-wallpaper. The "-" is because a panel matters more than
+# a wallpaper: nothing that script can do is allowed to stop the shell.
+ExecStartPre=-/usr/local/bin/classroom-desktop-wallpaper