desktops: disable DM in mode=build after install

[igorpecovnik]

Summary

DM package postinst (gdm3/lightdm/sddm) auto-enables the service on install. In mode=build this leaves the DM enabled in the cached rootfs, which would cause a headless login race on first boot before armbian-firstlogin has created the user.

Fix: disable both the specific DM and display-manager.service right after the DM install when mode=build.

if [[ "$mode" == "build" ]]; then
    systemctl disable "$DESKTOP_DM" 2>/dev/null || true
    systemctl disable display-manager 2>/dev/null || true
fi

Re-enablement on first boot

Verified in armbian-firstlogin:

• lightdm: line 948 — creates symlink /etc/systemd/system/display-manager.service → lightdm.service
• gdm3: line 977 — creates symlink → gdm3.service
• sddm: line 1018 — systemctl enable --now sddm

All three paths run after user creation, so the DM starts only when a real user exists.

Test plan

• Build a desktop image with mode=build — systemctl is-enabled gdm3 shows disabled in the rootfs
• First boot: armbian-firstlogin creates user, enables + starts the DM
• No change to runtime module_desktops install (mode not set → DM stays enabled as before)

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 4816c220-e71a-41d9-84e9-77d2b98bf351

📥 Commits

Reviewing files that changed from the base of the PR and between 051cebf and dcfdcb8.

📒 Files selected for processing (1)

• tools/modules/desktops/module_desktops.sh

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch build-mode-disable-dm

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