Improve notebook output and make fit persistence explicit

[AndrewSazonov]

• Render project-save file trees as one compact notebook output.
• Resize Colab Plotly frames correctly after asynchronous rendering.
• Require explicit project saves after fitting while preserving CLI persistence.
• Suppress unnecessary warnings for experiments without linked structures.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.37%. Comparing base (883c384) to head (25a0b19).
⚠️ Report is 130 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #220      +/-   ##
===========================================
+ Coverage    82.26%   90.37%   +8.11%     
===========================================
  Files           88      284     +196     
  Lines         5451    24780   +19329     
  Branches       454     2910    +2456     
===========================================
+ Hits          4484    22395   +17911     
- Misses         802     1756     +954     
- Partials       165      629     +464     

Flag	Coverage Δ
integration	?
unittests	90.37% <100.00%> (+17.92%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/easydiffraction/__main__.py	96.89% <100.00%> (+22.98%)	⬆️
src/easydiffraction/analysis/analysis.py	93.65% <ø> (+26.33%)	⬆️
src/easydiffraction/core/validation.py	94.16% <ø> (+4.60%)	⬆️
...easydiffraction/datablocks/experiment/item/base.py	95.28% <ø> (ø)
src/easydiffraction/display/plotters/plotly.py	94.34% <ø> (+11.84%)	⬆️
src/easydiffraction/project/project.py	97.81% <100.00%> (+30.67%)	⬆️

... and 323 files with indirect coverage changes

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