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>

README.md

@@ -97,7 +97,7 @@ Already have a clone? Just run it directly:
 
 Not run by `bootstrap.sh`:
 
-- `coder/desktop/` — Coder template for coder.nic-oconnor.com: an XFCE
+- `coder/desktop/` — Coder template for coder.nic-oconnor.com: a KDE Plasma
   desktop (KasmVNC) pod on the homelab cluster that applies this repo via the
   dotfiles module. Push with `coder templates push desktop -d coder/desktop`;
   see its README.