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/basic_truss.py

@@ -95,7 +95,7 @@ def build_basic_truss() -> Project:
             name="Basic Truss",
             author="OpenSees Examples Manual - Example 1",
             description="3-bar asymmetric planar truss, linear static analysis.",
-            units=UnitSystem.SI_M_N,
+            units=UnitSystem.METRIC,
         ),
         ndm=2, ndf=2,
         coord_systems=[