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/elastic_frame.py

@@ -227,7 +227,7 @@ def build_elastic_frame() -> Project:
                 "3-story 3-bay 2D elastic frame, AISC W-shape sections, "
                 "gravity (distributed) + lateral (point) + 5-mode eigen."
             ),
-            units=UnitSystem.US_IN_KIP,
+            units=UnitSystem.IMPERIAL,
         ),
         ndm=2, ndf=3,
         coord_systems=[