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