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/portal_pushover.py

@@ -64,7 +64,7 @@ def build_portal_pushover() -> Project:
 
     return Project(
         meta=ProjectMeta(name="Portal Pushover", author="Ozan",
-                         units=UnitSystem.SI_M_N),
+                         units=UnitSystem.METRIC),
         ndm=3, ndf=6,
         nodes=[
             Node(id=1, name="Base-L", coords=(0.0, 0.0, 0.0),