Minor updates to config parameters

[gitttt-1234]

This PR updates a few config parameters to align with SLEAP conventions:

• crop_hw → crop_size: simplifies the config by using a single integer instead of a list.

• save_ckpt_pth → save_ckpt_dir + run_name: checkpoint paths are now organized under <save_ckpt_dir>/<run_name>, with a new folder created for each run.

[codecov]

Codecov Report

❌ Patch coverage is 88.37209% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.35%. Comparing base (ff91433) to head (98c65a9).
⚠️ Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
sleap_nn/training/model_trainer.py	85.71%	4 Missing ⚠️
sleap_nn/config/trainer_config.py	87.50%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (88.37%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #316      +/-   ##
==========================================
+ Coverage   95.28%   95.35%   +0.07%     
==========================================
  Files          49       49              
  Lines        6765     6959     +194     
==========================================
+ Hits         6446     6636     +190     
- Misses        319      323       +4     

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