Add support for loading legacy SLEAP metrics (<=v1.4.1)

[gitttt-1234]

Summary

• Adds a new load_legacy_metrics.py module to handle loading metrics files from SLEAP versions <=v1.4.1
• Fixes NumPy 2.x compatibility by using public API (read_array_header_1_0, read_array_header_2_0) instead of removed private functions (_check_version, _read_array_header)
• Updates configs.py to detect and load legacy metrics format, displaying them in the evaluation metrics GUI table
• Fixes timestamp parsing to fallback to extracting from file path when run_name doesn't contain the timestamp pattern
• Updates labels.py to use the new legacy metrics loader for CLI output

Test plan

• Load a legacy SLEAP model checkpoint (<=v1.4.1) in the GUI metrics viewer
• Verify metrics display correctly in the table (mAP, mAR, PCK, OKS, distance percentiles)
• Verify timestamp column shows correct date/time parsed from folder name
• Verify new sleap-nn format models still load correctly
• Run ruff check to confirm linting passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 11.32075% with 94 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.06%. Comparing base (5f6eb12) to head (b2c4a42).

Files with missing lines	Patch %	Lines
sleap/gui/learning/load_legacy_metrics.py	18.64%	48 Missing ⚠️
sleap/gui/learning/configs.py	2.94%	33 Missing ⚠️
sleap/info/labels.py	0.00%	13 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2480      +/-   ##
===========================================
- Coverage    59.40%   59.06%   -0.34%     
===========================================
  Files           68       69       +1     
  Lines        13232    13324      +92     
===========================================
+ Hits          7860     7870      +10     
- Misses        5372     5454      +82     

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