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/concrete04_cantilever.osmodel

@@ -4,7 +4,7 @@
     "name": "Concrete04 Cantilever",
     "description": "Single RC column cantilever using Concrete04 (Popovics) fiber section. Demonstrates the Concrete04 material end-to-end: schema → model → runner → results.",
     "author": "",
-    "units": "SI (m, N, kg, s, Pa)"
+    "units": "Metric (m, N, kg, s, Pa)"
   },
   "ndm": 2,
   "ndf": 3,
@@ -237,6 +237,7 @@
       "pattern_ids": [
         1
       ],
+      "pattern_factors": {},
       "n_steps": 10,
       "load_factor_increment": 0.1,
       "system": "BandGeneral",
@@ -254,6 +255,7 @@
       "pattern_ids": [
         2
       ],
+      "pattern_factors": {},
       "control_node": 2,
       "control_dof": 1,
       "target_disp": 0.15000000000000002,