feat: give the classroom a desktop instead of a shell

The whole appliance UI is now configuration for a desktop Debian already
ships: a Plasma 6.3 session with a colour scheme, a look and feel package
that builds the bar, window rules, and KDE's own Kiosk restrictions.

Two things were measured rather than assumed, and both changed a decision.

Plasma restarts a crashed panel three times a minute and then gives up,
which is the failure a classroom has already had. Four lines of systemd
drop-in make it unconditional; the panel then survived eight SIGKILLs a
second apart, back each time in 2.4s.

Plasma draws a minimised task and a background task identically, so the
five states the room needs cannot all be shown. Rather than write a task
manager, the classroom removes minimise: clicking a launcher always means
show me this.

Co-Authored-By: Claude Opus 5 <[email protected]>

config/icons/ekklesia-parent-call.svg

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Parent Call.
+
+     Deliberately not a colour tile. Parent Call is not one of the room's
+     applications and does not sit in the row with them, so it is drawn in
+     the interface's own accent as a line mark: present, reachable, and
+     visibly a different kind of thing from Wonder Ink and the browser. -->
+<svg xmlns="http://www.w3.org/2000/svg" width="128" height="128" viewBox="0 0 128 128">
+  <!-- Inset so the mark has the same optical size as the filled tiles
+       beside it in the bar; an outline drawn to the edges reads larger. -->
+  <g transform="translate(64 64) scale(0.62) translate(-64 -64)"
+     fill="none" stroke="#4FB3A8" stroke-width="8"
+     stroke-linecap="round" stroke-linejoin="round">
+    <path d="M40 58a24 24 0 0 1 48 0c0 18 6 24 9 28H31c3-4 9-10 9-28z"/>
+    <path d="M55 94a9 9 0 0 0 18 0"/>
+    <path d="M64 26v-8"/>
+    <path d="M100 30a30 30 0 0 1 8 20"/>
+    <path d="M28 30a30 30 0 0 0-8 20"/>
+  </g>
+</svg>