setup-desktop: install tigervnc-common for vncpasswd

The VNC sidecar builds its .vnc/passwd rfbauth file with `vncpasswd -f`
(vnc-manager.ts) — x11vnc alone doesn't ship vncpasswd. The GNOME-on-Xorg
rewrite dropped tigervnc, so the mirror couldn't create its password and
/desktop failed with "VNC password not configured". Add tigervnc-common back.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/setup-desktop.sh

@@ -21,8 +21,11 @@ sudo DEBIAN_FRONTEND=noninteractive apt install -y -qq \
   ubuntu-desktop \
   gdm3 \
   x11vnc \
+  tigervnc-common \
   dbus-x11
 echo "  Done."
+# tigervnc-common provides `vncpasswd`, which the VNC sidecar uses to write the .vnc/passwd rfbauth
+# file that x11vnc mirrors :0 with (see vnc-manager.ts). x11vnc alone does NOT ship vncpasswd.
 
 # --- Step 2: Force GDM onto Xorg + enable auto-login (x11vnc cannot mirror Wayland) ---
 echo "[2/5] Forcing Xorg session and auto-login in GDM..."