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/_ex4_portal2d_common.py

@@ -304,7 +304,7 @@ def build_ex4_portal2d_elastic_element() -> Project:
             name=ELASTIC_VARIANT.name,
             author="OpenSees Wiki / Silvia Mazzoni & Frank McKenna",
             description=ELASTIC_VARIANT.description,
-            units=UnitSystem.US_IN_KIP,
+            units=UnitSystem.IMPERIAL,
         ),
         ndm=2,
         ndf=3,
@@ -333,7 +333,7 @@ def build_ex4_portal2d_inelastic_section() -> Project:
             name=INELASTIC_SECTION_VARIANT.name,
             author="OpenSees Wiki / Silvia Mazzoni & Frank McKenna",
             description=INELASTIC_SECTION_VARIANT.description,
-            units=UnitSystem.US_IN_KIP,
+            units=UnitSystem.IMPERIAL,
         ),
         ndm=2,
         ndf=3,
@@ -383,7 +383,7 @@ def build_ex4_portal2d_inelastic_fiber_section() -> Project:
             name=FIBER_VARIANT.name,
             author="OpenSees Wiki / Silvia Mazzoni & Frank McKenna",
             description=FIBER_VARIANT.description,
-            units=UnitSystem.US_IN_KIP,
+            units=UnitSystem.IMPERIAL,
         ),
         ndm=2,
         ndf=3,