Keep catalog compact and preserve downloads

[debpalash]

Summary

• keep distro and release rows inside a bounded scrolling viewport
• run ISO downloads in a detached per-user systemd service
• restore persisted Bootable NDJSON progress after panel or shell reload
• keep device discovery from replacing active download state
• sanitize release filenames and document the session lifecycle

Safety

• no target is selected automatically
• downloads remain unprivileged
• removable-media planning, acknowledgement, and write gates are unchanged

Verification

• omarchy plugin validate .
• sh -n bootable-status bootable-download-session
• qmllint -I /usr/share/omarchy/shell Panel.qml Service.qml (exit 0)
• detached invalid-distribution probe observed active state followed by the persisted Bootable failure event