Fix blank Mulliken 3D viewer (geometry persistence + lazy render)

[jonathanschultzNU]

Summary

Fixes the blank molecule viewer in the Mulliken Populations accordion while keeping the charge table and bar chart working.

Root cause: the dedicated Mulliken viewer needs a full Molecule (coordinates), but history replay often had Mulliken charges in result.json without any reachable geometry. Rendering also happened while the accordion content was still hidden, which Voilà can skip for <script> execution.

Changes

• Geometry persistence — save_result(..., molecule=calc_mol) writes a top-level geometry field to result.json; mol_from_result_dir() and build_history_context() load it for history replay.
• Robust geometry resolution — _mulliken_molecule() falls back through ctx.molecule, mol_from_result_dir(), _analysis_displayed_molecule, and _molecule.
• Lazy render — populate caches _mulliken_pending_molecule; the authoritative draw happens in _on_mulliken_accordion_show when the panel is visible.
• Unavailable message — when coordinates cannot be resolved, the viewer slot shows an inline explanation instead of a blank area.
• Theme/backend refresh — _rerender_3d_views() now refreshes the Mulliken viewer alongside the other Analysis 3D viewers.
• Tests — regression coverage for lazy render, result.json geometry replay, unavailable message, and theme rerender.

Test plan

• pytest tests/test_mulliken_panel.py tests/test_results_storage.py tests/test_populations_overlay.py
• pytest tests/test_sp_analysis_history.py tests/test_theme_contrast.py
• Manual: run Single Point, open Mulliken accordion — 3D viewer shows with charge colours
• Manual: load History result — viewer renders without requiring Calculate-tab molecule
• Manual: toggle Light/Dark theme — Mulliken viewer background updates