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/sdof_pushover.py

@@ -48,7 +48,7 @@ from otko.services import load_project, save_project
 def build_sdof() -> Project:
     return Project(
         meta=ProjectMeta(name="SDOF Pushover", author="Ozan",
-                         units=UnitSystem.SI_M_N),
+                         units=UnitSystem.METRIC),
         ndm=3, ndf=6,
         nodes=[
             Node(id=1, name="Base", coords=(0.0, 0.0, 0.0),