Move to Ubuntu 26.04 LTS: backup, setup and restore scripts

Replaces the Bazzite/BlueBuild image with plain Ubuntu. apt covers the system
and dev tools, Flathub the desktop apps, and three scripts do the rest:
backup.sh archives this machine's home to server-marvin and reads it back to
verify it; setup.sh builds a fresh 26.04 install (NVIDIA 595-open, Docker with
GPU access, Steam/Lutris, 27 Flatpaks, VS Code, Claude desktop, the Data drive
and marvin mounts); restore.sh unpacks the backup without the Fedora/KDE parts
that would fight Ubuntu.

The app list comes from a scan of the running machine. The 15 Flatpaks left
behind sit commented at the bottom of flatpaks.txt, and their data still rides
along in the backup. No Node or .NET on the host by choice — LudosData and
landingPage build in Docker.

Drops the BlueBuild recipe, the cosign key and the old dev-setup script. The
Bazzite setup stays in this history.

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

.gitignore

@@ -1,3 +0,0 @@
-# NEVER commit the cosign private key — it goes in the SIGNING_SECRET GitHub secret.
-cosign.key
-/.bluebuild-scripts_*