Followup for "Session: create installer systemd service"

[ryonakano]

Additional work for #929 to work. This should also fixes the current Daily OS 9 iso directly goes to the demo session instead of the installer session.

Changes summary

• Use gala instead of the greeter compositor (which Danielle wrote as TODO in the above PR)
• Rename io.elementary.installer.compositor.service to io.elementary.installer.compositor@wayland.service to make sure ConditionEnvironment with unescaped instance name %I works
• Merge with gala@wayland.service.in because I guess it was based on gala@x11.service.in
• Fix missing session.conf
  • This was actually needed to make things work, as also written in the doc: https://gnome.pages.gitlab.gnome.org/gnome-session/re01.html
  • The content is based on RequiredComponents in session/installer.session
• Change relationship between io.elementary.installer.target and io.elementary.installer.service so that Installer launches when switching to the installer session
  • Before: The .service is WantedBy the .target
  • After: The .target Wants the .service

Checklists

• Clicking the Installer icon in the dock goes to the installer session and Installer appears

Screencast.From.2026-08-11.14-17-32.mp4

Note: You'll need elementary/gala#2889 to make Wingpanel appear in the installer session.

[danirabbit]

Thanks for fixing this!