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/plasma/lookandfeel/metadata.json

@@ -0,0 +1,17 @@
+{
+    "KPackageStructure": "Plasma/LookAndFeel",
+    "KPlugin": {
+        "Authors": [
+            {
+                "Name": "Ekklesia"
+            }
+        ],
+        "Category": "",
+        "Description": "The classroom appliance desktop",
+        "Id": "org.ekklesia.classroom.desktop",
+        "License": "GPLv2+",
+        "Name": "Ekklesia Classroom",
+        "Website": "https://riverofglorymn.org"
+    },
+    "X-Plasma-APIVersion": "2"
+}