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

tests/unit/test_phase8b.py

@@ -89,7 +89,7 @@ def test_pushover_case_schema() -> None:
 
 def test_pushover_case_round_trip(tmp_path) -> None:  # type: ignore[no-untyped-def]
     p = Project(
-        meta=ProjectMeta(name="PO test", units=UnitSystem.SI_M_N),
+        meta=ProjectMeta(name="PO test", units=UnitSystem.METRIC),
         ndm=3, ndf=6,
         nodes=[
             Node(id=1, coords=(0, 0, 0), restraint=(True,) * 6),
@@ -144,7 +144,7 @@ def test_pushover_case_round_trip(tmp_path) -> None:  # type: ignore[no-untyped-
 
 def test_beam_with_hinges_round_trip(tmp_path) -> None:  # type: ignore[no-untyped-def]
     p = Project(
-        meta=ProjectMeta(name="BWH", units=UnitSystem.SI_M_N),
+        meta=ProjectMeta(name="BWH", units=UnitSystem.METRIC),
         ndm=3, ndf=6,
         nodes=[
             Node(id=1, coords=(0, 0, 0), restraint=(True,) * 6),