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_phase8c.py

@@ -87,7 +87,7 @@ def _two_dof_modal(masses: list[float]) -> tuple[Project, ModalResults]:
     φ = (1, -1). Eigenvalues set to ω² = 100 and 400 (T = 0.628 s, 0.314 s).
     """
     p = Project(
-        meta=ProjectMeta(name="2dof", units=UnitSystem.SI_M_N),
+        meta=ProjectMeta(name="2dof", units=UnitSystem.METRIC),
         ndm=3, ndf=6,
         nodes=[
             Node(id=1, coords=(0, 0, 0), restraint=(True,) * 6),
@@ -170,7 +170,7 @@ def test_cqc_equals_srss_for_well_separated_modes() -> None:
 # ── ResponseSpectrumCase persistence ─────────────────────────────────
 def test_response_spectrum_case_round_trip(tmp_path) -> None:  # type: ignore[no-untyped-def]
     p = Project(
-        meta=ProjectMeta(name="rs", units=UnitSystem.SI_M_N),
+        meta=ProjectMeta(name="rs", units=UnitSystem.METRIC),
         ndm=3, ndf=6,
         nodes=[Node(id=1, coords=(0, 0, 0), restraint=(True,) * 6),
                Node(id=2, coords=(0, 0, 3))],