Add Ubuntu 26.04 virtual machine

Add run-ubuntu2604.py with its QEMU config files and NoCloud seed. The
launcher follows the Debian 13 one, with UBUNTU_VM_ settings and SSH
forwarded to host port 2225. The seed injects the current user's SSH public
key, uses the USTC mirror (ubuntu-ports on arm64) and installs the GNOME
desktop on first boot. Update vm-images for the ubuntu2604 image target.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kwxp2XWvKnuEBifnCbhyX5

qemu/ubuntu2604-arm64.ini

@@ -0,0 +1,17 @@
+# arm64 part of the Ubuntu 26.04 QEMU configuration; read after ubuntu2604.ini.
+
+[machine]
+  type = "virt"
+
+# Unlike q35, the "virt" machine has no default GPU or USB input devices.
+[device "gpu0"]
+  driver = "virtio-gpu-pci"
+
+[device "usb0"]
+  driver = "qemu-xhci"
+
+[device "kbd0"]
+  driver = "usb-kbd"
+
+[device "tablet0"]
+  driver = "usb-tablet"