refactor(structure): extract constants, fix cyclic imports, improve repr/docs/tests

- Extract shared constants into src/aiida_atomistic/data/structure/constants.py and
  update imports across the package to break a circular import chain that caused
  Sphinx AutoAPI warnings.
- Make `Site.__repr__` robust to missing positions to avoid TypeError in notebooks.
- Rework `Structure.__repr__` and add `__str__` to present a safe, informative
  summary without calling `super().__repr__()` (prevents recursion with AiiDA Node repr).
- Update `utils.sites_from_kinds` to accept both list and dict kinds formats for
  more flexible initialization.
- Add comprehensive tests for property setters/removers: tests/data/test_property_setters.py.
- Document remove-property methods and clarify property-setting vs validation semantics;
  clarify `StructureBuilder` role; document that `magmom` and `magnetization` are
  mutually exclusive per site.
- Minor validator and utility fixes (freeze_pbc validator ordering, small import fixes).

Why:
- Removes AutoAPI cyclic import warnings and prevents runtime repr-related errors
  in notebooks and interactive sessions.
- Improves developer ergonomics and documents expected behaviour for magnetic
  properties and property removal.

Testing:
- Run unit tests: pytest -q
- Rebuild docs: cd docs && make html

Notes:
- Internal refactor only: no intended public API breaking changes. Maintainers
  should note the new `constants.py` location for internal imports.

Author: mikibonacci

conftest.py

@@ -97,14 +97,14 @@
 myst_substitutions = {
     'aiida_logo': '<img src="../_static/logo_aiida.svg" alt="aiida" class="aiida-logo">',
     'StructureData': '{class}`~aiida_atomistic.data.structure.structure.StructureData`',
-    'StructureDataMutable': '{class}`~aiida_atomistic.data.structure.structure.StructureDataMutable`',
+    'StructureBuilder': '{class}`~aiida_atomistic.data.structure.structure.StructureBuilder`',
     'Site': '{class}`~aiida_atomistic.data.structure.site.Site`',
     'Kind': '{class}`~aiida_atomistic.data.structure.kind.Kind`',
     'LegacyStructureData': '{class}`~aiida.orm.StructureData`',
     'from_ase': '{meth}`~aiida_atomistic.data.structure.structure.StructureData.from_ase`',
     'from_pymatgen': '{meth}`~aiida_atomistic.data.structure.structure.StructureData.from_pymatgen`',
     'from_file': '{meth}`~aiida_atomistic.data.structure.structure.StructureData.from_file`',
-    'from_legacy': '{meth}`~aiida_atomistic.data.structure.structure.StructureData.from_legacy`',
+    'legacy_structure.to_atomistic()': '{meth}`~aiida_atomistic.data.structure.structure.StructureData.legacy_structure.to_atomistic()`',
 }
 
 # -- Options for HTML output ----------------------------------------------
@@ -119,6 +119,8 @@
     'twitter_url': 'https://twitter.com/aiidateam',
     'use_edit_page_button': True,
     'navigation_with_keys': False,
+    'show_navbar_depth': 2,  # Show 2 levels in navbar
+    'show_toc_level': 2,  # Show 2 levels in right sidebar TOC
     'logo': {
         'text': 'AiiDA Atomistic',
         'image_light': '_static/logo_aiida_atomistic-light.png',