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_frame.py

@@ -59,7 +59,7 @@ def _sine_pulse_factors() -> list[float]:
 def build_portal_frame() -> Project:
     """A two-column, one-beam steel portal frame with static + dynamic cases."""
     return Project(
-        meta=ProjectMeta(name="Portal Frame", author="Ozan", units=UnitSystem.SI_M_N),
+        meta=ProjectMeta(name="Portal Frame", author="Ozan", units=UnitSystem.METRIC),
         ndm=3,
         ndf=6,
         nodes=[