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/beam_quad_2d.py

@@ -132,7 +132,7 @@ def build_beam_quad_2d() -> Project:
                 f"{NX}×{NY} plane-stress quad mesh of a {L:.0f}×{H:.0f} deep "
                 "beam with two midspan point loads, LoadControl 0→10 in 10 steps."
             ),
-            units=UnitSystem.US_IN_KIP,
+            units=UnitSystem.IMPERIAL,
         ),
         ndm=2, ndf=2,
         nodes=nodes,