Fix invalid CSS properties breaking assistant toggle on wide displays

[Copilot]

Motivation

The assistant toggle button was not properly positioned on displays ≥1400px due to invalid CSS properties max-margin-right and max-right in the media query for large screens.

How to test the behavior?

1. Build the documentation: cd docs && make html
2. Open docs/_build/html/index.html in a browser
3. Resize browser window to ≥1400px width
4. Click "Open Assistant" button
5. Verify the toggle button stays aligned with the assistant panel edge
6. Resize to ≥2000px and verify panel width caps at 600px

Or test on the live dev docs at https://pynwb.readthedocs.io/en/dev/

Changes

Before: Used invalid CSS properties that browsers ignore

.assistant-container.show ~ .assistant-toggle {
    right: calc(100vw - 1140px);
    max-right: 600px;  /* Invalid property */
}

After: Use CSS min() function for proper constraint

.assistant-container.show ~ .assistant-toggle {
    right: min(calc(100vw - 1140px), 600px);
}

Applied same fix to width, margin-right, and right properties in the @media (min-width: 1400px) query.

Screenshots

1600px viewport - dynamic sizing:

2000px viewport - capped at 600px:

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff check . && codespell from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Bug]: Floating open/close assistant tab for wider displays</issue_title>
<issue_description>### What happened?

See comment in #2140 (comment).

Steps to Reproduce

Use the dev build of the documentation on a larger display: https://pynwb.readthedocs.io/en/dev/

Operating System

macOS

Python Executable

Conda

Python Version

3.9

Package Versions

No response

Code of Conduct

• I agree to follow this project's Code of Conduct
• Have you checked the Contributing document?
• Have you ensured this bug was not already reported?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug]: Floating open/close assistant tab for wider displays #2145

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.23%. Comparing base (7e100c2) to head (5335ceb).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #2151   +/-   ##
=======================================
  Coverage   95.23%   95.23%           
=======================================
  Files          29       29           
  Lines        2876     2876           
  Branches      740      740           
=======================================
  Hits         2739     2739           
  Misses         83       83           
  Partials       54       54           

Flag	Coverage Δ
integration	74.44% <ø> (ø)
unit	84.94% <ø> (ø)

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[rly]

@copilot Update the changelog with a link to this PR #2151

[Copilot]

@copilot Update the changelog with a link to this PR #2151

Updated in commit dba2312.