start-umbriel: dbus-run-session if needed

[EmiOnFlux]

Summary

Let start-umbriel start a dbus session if one it not already running.

Motivation

Avoid the need for non systemd users to manually have to edit their desktop file to append dbus-run-session to the exec line of the desktop file

Type of Change

• Bug fix (Non SystemD)
• New feature
• Breaking change
• Refactoring
• Build / packaging
• Documentation

Manual Coverage

• Tested in a nested Umbriel session
• Tested in a native Umbriel session
• Tested with multiple monitors
• Tested with a scaled output
• Tested with native Wayland applications
• Tested with X11 applications through xwayland-satellite
• Tested with the scrolling layout
• Tested with the dwindle layout

Screenshots / Videos

Testing

just running the compositor with start-umbriel

Checklist

• This PR is ready for review, or it is marked as Draft.
• This change fits SCOPE.md, or its scope was agreed in an issue or on Discord first.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format, or this PR has no C++ changes.
• I ran the relevant build, test, lint, or verification commands, or explained why they were not run.
• I functionally verified compositor behavior where automated checks are insufficient.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I updated docs/ and examples/config.toml, or this PR does not change user-facing configuration or behavior.
• I used canonical names for config keys, IPC actions, paths, and identifiers.

[github-actions]

This pull request was automatically closed because its description no longer contains
every part of the pull request template
that this repository requires.

Missing:

• the template marker line <!-- umbriel-pr-template:v1 -->
• the ## Related Issue heading
• the ## Testing heading
• the ## Additional Notes heading
• the checked checklist entry: I functionally verified compositor behavior where automated checks are insufficient.
• the checked checklist entry: I used canonical names for config keys, IPC actions, paths, and identifiers.

Please add the items listed above back to the description, keeping their exact wording, then
reopen the pull request. Reopening re-runs this check. Draft pull requests may leave boxes
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[github-actions]

This pull request was converted to a draft because its description is missing required
parts of the pull request template.

Missing:

• the ## Testing heading

Add the items above to the description, keeping their exact wording, then mark the pull
request ready for review. That re-runs this check. Draft pull requests may leave boxes
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check every item under Checklist.

Sections that only offer context may be deleted; nothing else about this pull request was
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[github-actions]

The description now contains the required template structure.

[Ly-sec]

Thank you very much @EmiOnFlux , merging it :).