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/eigen_two_storey_shear_frame.osmodel

@@ -4,7 +4,7 @@
     "name": "Eigen - Two-Storey Shear Frame",
     "description": "Two-storey shear frame with equalDOF floor constraints and two-mode eigenvalue analysis.",
     "author": "OpenSees Wiki / Chopra Example 10.4",
-    "units": "US (in, kip, kip·s²/in, s, ksi)"
+    "units": "Imperial (in, kip, kip·s²/in, s, ksi)"
   },
   "ndm": 2,
   "ndf": 3,
@@ -198,7 +198,8 @@
       "Iz": 640.0,
       "Iy": null,
       "G": null,
-      "J": null
+      "J": null,
+      "shape": null
     },
     {
       "id": 2,
@@ -209,7 +210,8 @@
       "Iz": 320.0,
       "Iy": null,
       "G": null,
-      "J": null
+      "J": null,
+      "shape": null
     },
     {
       "id": 3,
@@ -220,7 +222,8 @@
       "Iz": 10000000000000.0,
       "Iy": null,
       "G": null,
-      "J": null
+      "J": null,
+      "shape": null
     }
   ],
   "elements": [
@@ -234,7 +237,8 @@
       ],
       "section_id": 1,
       "geom_transf": "Linear",
-      "rho": 0.0
+      "rho": 0.0,
+      "consistent_mass": false
     },
     {
       "id": 2,
@@ -246,7 +250,8 @@
       ],
       "section_id": 2,
       "geom_transf": "Linear",
-      "rho": 0.0
+      "rho": 0.0,
+      "consistent_mass": false
     },
     {
       "id": 3,
@@ -258,7 +263,8 @@
       ],
       "section_id": 1,
       "geom_transf": "Linear",
-      "rho": 0.0
+      "rho": 0.0,
+      "consistent_mass": false
     },
     {
       "id": 4,
@@ -270,7 +276,8 @@
       ],
       "section_id": 2,
       "geom_transf": "Linear",
-      "rho": 0.0
+      "rho": 0.0,
+      "consistent_mass": false
     },
     {
       "id": 5,
@@ -282,7 +289,8 @@
       ],
       "section_id": 3,
       "geom_transf": "Linear",
-      "rho": 0.0
+      "rho": 0.0,
+      "consistent_mass": false
     },
     {
       "id": 6,
@@ -294,7 +302,8 @@
       ],
       "section_id": 3,
       "geom_transf": "Linear",
-      "rho": 0.0
+      "rho": 0.0,
+      "consistent_mass": false
     }
   ],
   "mp_constraints": [