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/basic_truss.osmodel

@@ -4,7 +4,7 @@
     "name": "Basic Truss",
     "description": "3-bar asymmetric planar truss, linear static analysis.",
     "author": "OpenSees Examples Manual - Example 1",
-    "units": "SI (m, N, kg, s, Pa)"
+    "units": "Metric (m, N, kg, s, Pa)"
   },
   "ndm": 2,
   "ndf": 2,
@@ -245,6 +245,7 @@
       "rho": 0.0
     }
   ],
+  "mp_constraints": [],
   "time_series": [
     {
       "id": 1,
@@ -284,6 +285,7 @@
       "pattern_ids": [
         1
       ],
+      "pattern_factors": {},
       "n_steps": 1,
       "load_factor_increment": 1.0,
       "system": "BandGeneral",