fix: set SHOW_AXES to False by default

[VPK570]

What is this PR:

• Bug fix

Why is this PR needed?
The axes are shown by default but don't need to be.
Users who don't want axes currently have to explicitly
set settings.SHOW_AXES = False in their code.

What does this PR do?
Sets SHOW_AXES to False in brainrender/settings.py so
axes are hidden by default.

References:
Fixes #441

How has this PR been tested?
Changed the setting locally and verified the grep output
confirms SHOW_AXES = False in settings.py. The change is
a single boolean value with no logic implications beyond
what is already tested via the existing render.py usage.

Is this a breaking change?
This could be considered a minor breaking change for users
who rely on axes being shown by default. They will need to
explicitly set settings.SHOW_AXES = True.

Does this PR require an update to the documentation?
The examples/settings.py already demonstrates how to toggle
SHOW_AXES, so no documentation update is needed.

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.60%. Comparing base (62a434f) to head (2583e65).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #443   +/-   ##
=======================================
  Coverage   86.60%   86.60%           
=======================================
  Files          27       27           
  Lines        1239     1239           
=======================================
  Hits         1073     1073           
  Misses        166      166           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.