message_view_header: Add description to views in top-navbar.

[Mahhheshh]

fixes: #29769

• Add Help Center Link.

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Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[zulipbot]

Hello @zulip/server-onboarding members, this pull request was labeled with the "area: onboarding" label, so you may want to check it out!

[Mahhheshh]

removed because of the;

[alya]

I'm not sure why we have 2 PRs for this issue, but please seem #29824 and my comments on it, in case they are relevant.