Add theme: Lock Pinned Tabs

[mr-cheffy]

Add theme: Lock Pinned Tabs

This PR adds a new theme to the theme library.

Theme Details

• Name: Lock Pinned Tabs
• Description: Hides the close × on pinned tabs so an accidental click can't dismiss them.
• Homepage: https://github.com/rocavence/zen-workspace-tabs-as-assets
• Author: @rocavence

[rocavence]

Hi, thanks for setting up the PR! Two small heads-ups about the source repo since I submitted:

• Relicensed to MIT (it was CC BY-NC-SA before) so it's properly open-source.
• Renamed the repo to match the theme name: rocavence/zen-workspace-tabs-as-assets → rocavence/lock-pinned-tabs. The old URL 301-redirects to the new one, so the homepage in this PR still resolves correctly — no change needed on your side.

Nothing else changed (the CSS and image are the same). Happy to adjust anything if needed. Thanks!