Fix issues with serve and URL of live preview

[AgentHagu]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
Fixes #2617
Fixes #2620

Anything you'd like to highlight/discuss:
nil

Testing instructions:
To test fixes for #2617:

1. Serve a markbind site using an IPv6 address, such as markbind serve -a ::1.
2. The URL opened should be something like http://[::1]:8080/, i.e. the IPv6 address is wrapped in square brackets to form a valid URL. IPv4 addresses should not be affected/adjusted.

To test fixes for #2620:

1. Serve a MarkBind site using an unavailable address, such as:

   • markbind serve -a 1.1.1.1
   • markbind serve -a 192.168.1.0

   Alternatively, send the same command with an originally available address twice, effectively making that address unavailable for the second serve command.

2. The URL opened after serving the site should be the one the MarkBind site is actually hosted on, not the unavailable address provided in the command.

Proposed commit message: (wrap lines at 72 characters)
Fix issues with serve and URL of live preview

Add square brackets [] to IPv6 addresses and fixes issue regarding
unavailable address and opening the wrong URL.

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

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Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.86%. Comparing base (8c9de42) to head (1c5c9cd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2621   +/-   ##
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  Coverage   51.86%   51.86%           
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  Files         127      127           
  Lines        5474     5474           
  Branches     1201     1201           
=======================================
  Hits         2839     2839           
  Misses       2340     2340           
  Partials      295      295           

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