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/moment_curvature.py

@@ -14,7 +14,7 @@ linear reference moment pattern (Mz = 1 kip·in) is added and
 DisplacementControl on DOF 3 ramps the curvature to μ·Ky where
 μ = 15 and Ky is the elastic yield curvature estimate.
 
-Units: kip, in, ksi (UnitSystem.US_IN_KIP).
+Units: kip, in, ksi (UnitSystem.IMPERIAL).
 
 GUI walkthrough: File → Open → moment_curvature.osmodel, Options →
 Set Display Units → US (in, kip, kip·s²/in, s, ksi), Analyze → Run →
@@ -83,7 +83,7 @@ def build_moment_curvature() -> Project:
                 "RC column fibre section — constant axial P + DisplacementControl "
                 "curvature pushover. Kip-in-ksi units throughout."
             ),
-            units=UnitSystem.US_IN_KIP,
+            units=UnitSystem.IMPERIAL,
         ),
         ndm=2, ndf=3,
         coord_systems=[