coder/desktop: KDE Plasma instead of XFCE

Add image/Dockerfile (published as
git.nic-oconnor.com/public/coder-desktop-kde:noble): enterprise-desktop
noble plus KDE Plasma and KasmVNC 1.5.0, baked in so starts skip apt.
It carries three fixes found testing in the cluster:

- kwin-x11 installed explicitly (only a Recommends; without it Plasma
  runs with no window manager),
- startkde -> startplasma-x11 symlink, since KasmVNC's -select-de kde
  checks for startkde and Plasma 5.20+ renamed it,
- KWin compositing off by default (no GPU; it only adds VNC lag).

The template now uses that image with desktop_environment = "kde" and
pulls Always, and drops the XFCE .xsessionrc seeding.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

coder/desktop/image/Dockerfile

@@ -0,0 +1,34 @@
+# coder/desktop/image — the workspace image: Coder's desktop base with KDE
+# Plasma and KasmVNC baked in, so a start does not spend minutes in apt (the
+# root filesystem is not persisted; only /home/coder is).
+#
+#   docker build -t git.nic-oconnor.com/public/coder-desktop-kde:noble coder/desktop/image
+#   docker push git.nic-oconnor.com/public/coder-desktop-kde:noble
+#
+# noble, not resolute: KasmVNC publishes no resolute package yet.
+FROM codercom/enterprise-desktop:ubuntu-noble
+
+ARG KASMVNC_VERSION=1.5.0
+
+USER root
+# kwin-x11 is only a Recommends of kde-plasma-desktop, so --no-install-recommends
+# drops it and Plasma comes up with no window manager (no title bars).
+RUN apt-get update \
+ && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+      kde-plasma-desktop kwin-x11 dolphin konsole kate plasma-nm breeze-gtk-theme \
+      dbus-x11 libdatetime-perl \
+ && curl -fsSL -o /tmp/kasmvnc.deb \
+      "https://github.com/kasmtech/KasmVNC/releases/download/v${KASMVNC_VERSION}/kasmvncserver_noble_${KASMVNC_VERSION}_amd64.deb" \
+ && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends /tmp/kasmvnc.deb \
+ && rm -f /tmp/kasmvnc.deb /home/coder/.xsessionrc \
+ && rm -rf /var/lib/apt/lists/*
+
+# KasmVNC's `-select-de kde` (what the Coder kasmvnc module runs) looks for a
+# `startkde` binary, which Plasma 5.20+ renamed to startplasma-x11.
+RUN ln -s /usr/bin/startplasma-x11 /usr/local/bin/startkde
+
+# No GPU in the pod: software compositing over VNC only adds lag. Users can
+# still turn it on in System Settings (~/.config/kwinrc overrides this).
+RUN printf '[Compositing]\nEnabled=false\n' > /etc/xdg/kwinrc
+
+USER coder