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/rc_frame_gravity.osmodel

@@ -4,7 +4,7 @@
     "name": "RC Frame Gravity (OpenSees Ex 3)",
     "description": "1-bay 1-storey portal frame, nonlinear fibre columns + elastic beam, 10-step LoadControl gravity pushdown.",
     "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,
@@ -313,7 +313,8 @@
       "Iz": 8640.0,
       "Iy": 8640.0,
       "G": 1500.0,
-      "J": 1.0
+      "J": 1.0,
+      "shape": null
     }
   ],
   "elements": [
@@ -355,9 +356,11 @@
       ],
       "section_id": 2,
       "geom_transf": "Linear",
-      "rho": 0.0
+      "rho": 0.0,
+      "consistent_mass": false
     }
   ],
+  "mp_constraints": [],
   "time_series": [
     {
       "id": 1,
@@ -408,6 +411,7 @@
       "pattern_ids": [
         1
       ],
+      "pattern_factors": {},
       "n_steps": 10,
       "load_factor_increment": 0.1,
       "system": "BandGeneral",