PR-R3: Persist viewer representations in extras

[cpignedoli]

What

PR-R3 in the representation split. This is stacked on top of #769.

This PR adds the persistence side of viewer representations:

• convert viewer representation arrays into JSON-safe masks;
• store those masks in StructureData/CifData extras;
• add a Store representations button for already-stored structures;
• make Store in AiiDA persist representation masks when a structure is first stored;
• keep standalone viewer Apply representations in sync with extras when the viewer has a stored node.

Why

Users can now change visualization styles without creating a new structure node. The geometry remains the same AiiDA node, while the mutable viewer state lives in extras.

Small defaulting fix

This PR also makes the StructureManagerWidget storage format selector explicitly default to StructureData when no node_class is passed.

Previously the RadioButtons widget relied on the first entry in SUPPORTED_DATA_FORMATS. That is implicit and can silently select the wrong storage type if the option order changes. The new logic keeps explicit caller choices unchanged, but falls back to StructureData for the default structure-manager path. This matters here because representation persistence is stored on the AiiDA node extras, so the node type must be the one the widget is actually intended to create.

Chain

• PR-R1: Encode structure viewer representation styles #768, encoded representation style IDs
• PR-R2: PR-R2: Load viewer representations from extras #769, load representation masks from extras and drop stale rows
• PR-R3: this PR, persist representation masks back to extras

Implemented with Codex.

Validation

python -m pytest tests/test_viewers.py tests/test_structures.py

Result: 37 passed, 24 warnings.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 5 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (feature/load-viewer-representations-from-extras@2ea5cd3). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
aiidalab_widgets_base/structures.py	80.95%	4 Missing ⚠️
aiidalab_widgets_base/viewers.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@                                Coverage Diff                                 @@
##             feature/load-viewer-representations-from-extras     #770   +/-   ##
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  Coverage                                                   ?   85.69%           
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  Files                                                      ?       18           
  Lines                                                      ?     3426           
  Branches                                                   ?        0           
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  Hits                                                       ?     2936           
  Misses                                                     ?      490           
  Partials                                                   ?        0           

Flag	Coverage Δ
python-3.12	85.66% <85.71%> (?)
python-3.9	85.62% <85.71%> (?)

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