feat: consolidate units to Metric/Imperial with unit-aware dialogs

- core/units: two dominant systems (Metric m/kN, Imperial ft/kip) with
  display conversion helpers, legacy 4-system migration in ProjectMeta
- dialogs/docks: unit-aware material, section, case, load, grid and
  results labels; diagram renderer unit labels; render controls update
- docs: add consistent_units.md; regen examples/*.osmodel artifacts
- tests: update persistence/phase8/project/unit-labels for new systems

examples/ex2c_canti2d_inelastic_fiber_section.py

@@ -120,7 +120,7 @@ def build_ex2c_canti2d_inelastic_fiber_section() -> Project:
                 "Nonlinear cantilever with a fiber section built from "
                 "Concrete02 and Steel02 uniaxial materials."
             ),
-            units=UnitSystem.US_IN_KIP,
+            units=UnitSystem.IMPERIAL,
         ),
         ndm=2,
         ndf=3,