feat(gui): improve CLI help output and argument validation

[joshtrichards]

Summary

Make the GUI's command-line interface easier to use by improving the help text and tightening argument parsing.

Changes

• rewrite the --help output into clearer grouped sections
• clarify descriptions for setup wizard and non-interactive provisioning options
• refactor: group related options logic more clearly in parseOptions()
• refactor: simplify option matching in parseOptions() with small local helpers
• report missing values more consistently for value-taking options
• reject invalid --logexpire values instead of silently treating them as 0
• document known follow-up areas around --reverse and --overridelocaldir

Notes

• this is intended as a low-risk cleanup of CLI help and parsing logic
• delegated command-line parsing via AccountSetupCommandLineManager is unchanged
• no intended behavior changes beyond clearer errors and stricter --logexpire validation

Follow-up

Potential follow-ups kept out of this PR:

• decide whether --reverse should set RTL unconditionally
• validate the path passed to --overridelocaldir
• revisit whether path-like option values beginning with -- should be accepted
• validate what --background does and does not do and set expectations accordingly in the help text/etc and address [Bug]: --background not working #5916

Checklist

• Sign-off message is added to all commits.
• The commit history is clean with no merge commits.
  • How to rebase your commits
  • How to squash commits with rebase
• Uploaded screenshots from before and after for UI changes.
• Documentation has been updated or is not required.
• Backports requested where applicable (critical bugfixes).
• Labels added where applicable (bug/enhancement).
• Milestone added for target version.

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)
22 New Code Smells (required ≤ 0)
D Security Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-10069.zip

Digest: sha256:da223dddf688f2278cbf412b02a858509e2d1f4120b7f1977698784d6dc19e95

To test this change/fix you can download the above artifact file, unzip it, and run it.

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