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/chrome/policies/managed/ekklesia-classroom.json

@@ -2,25 +2,20 @@
   "DefaultBrowserSettingEnabled": false,
   "PromotionalTabsEnabled": false,
   "MetricsReportingEnabled": false,
-
   "BrowserSignin": 0,
   "SyncDisabled": true,
   "SigninInterceptionEnabled": false,
-
   "BrowserAddPersonEnabled": false,
   "BrowserGuestModeEnabled": false,
   "ProfilePickerOnStartupAvailability": 1,
-
   "PasswordManagerEnabled": false,
   "PasswordLeakDetectionEnabled": false,
   "BackgroundModeEnabled": false,
   "DisablePrintPreview": true,
-
   "CookiesSessionOnlyForUrls": [
     "[*.]wonderink.org"
   ],
-
   "ShowHomeButton": true,
   "HomepageIsNewTabPage": false,
-  "HomepageLocation": "https://app.wonderink.org/"
+  "HomepageLocation": "https://www.youtube.com/"
 }