feat(gnome): Enable systemd-homed support by default

[castrojo]

Rebased version of #962 by @dylanmtaylor — all credit to the original author.

PR #962 became noisy because the next branch was heavily advanced with CI changes after @dylanmtaylor forked, making the diff appear to remove 30+ workflow files. The actual contribution is one file.

What

Integrates a patch to enable systemd-homed unconditionally across the GNOME stack:

• Enabling -Dcreate_homed=true in accountsservice
• Removing malcontent dependencies and disabling parental controls from gnome-control-center, gnome-initial-setup, and gnome-software
• Configuring the live generator to set DefaultStorage=subvolume for homed home directories

Port of gnome-build-meta!2681.

Why this PR instead of #962

#962 was opened against an older base and now shows ~30 workflow files as "removed" — that's CI drift noise, not the contributor's change. This PR is cherry-picked to current next and shows only the actual patch.

Closes #962

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[github-actions]

This PR targets next instead of main. Please retarget it to main — the next branch is managed by the promotion pipeline and must not receive direct PRs.

[castrojo]

Duplicate of #963 which has a BST build already running on the lab. Closing this one.