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.osmodel

@@ -4,7 +4,7 @@
     "name": "Moment-Curvature (OpenSees Ex 2)",
     "description": "RC column fibre section — constant axial P + DisplacementControl curvature pushover. Kip-in-ksi units throughout.",
     "author": "OpenSees Examples Manual",
-    "units": "US (in, kip, kip·s²/in, s, ksi)"
+    "units": "Imperial (in, kip, kip·s²/in, s, ksi)"
   },
   "ndm": 2,
   "ndf": 3,
@@ -251,6 +251,7 @@
       "section_id": 1
     }
   ],
+  "mp_constraints": [],
   "time_series": [
     {
       "id": 1,
@@ -317,6 +318,7 @@
         1,
         2
       ],
+      "pattern_factors": {},
       "control_node": 2,
       "control_dof": 3,
       "target_disp": 0.0019047619047619052,
@@ -329,7 +331,8 @@
       "algorithm": "Newton",
       "test": "NormUnbalance",
       "tolerance": 1e-09,
-      "max_iter": 25
+      "max_iter": 25,
+      "preload_case_ids": []
     }
   ]
 }