refactor(gui): features register their own settings surfaces

Settings collaboration direction (recorded in the note): the shell only
provides composition faces — feature plugins register themselves. The
General section moves into the ui-settings shell (order 0, skeleton
rows) and declares the settings.general.item list slot; locale registers
the Language row and ui-theme the Appearance row (each with its own
store mirror, dictionaries, and ledger-judged deferral); the
ui-settings-general package is gone. ui-settings-models becomes
ui-models — a feature package that contributes its Settings section
rather than a settings-owned satellite. The item-slot SlotMap entry is
authored in the ui-settings contract and repeated verbatim in
locale/ui-theme (reference-cycle avoidance; declaration merging keeps
the copies identical).

packages/client/ui-theme/tsconfig.json

@@ -8,6 +8,18 @@
     "src"
   ],
   "references": [
+    {
+      "path": "../locale"
+    },
+    {
+      "path": "../runtime"
+    },
+    {
+      "path": "../ui-primitives"
+    },
+    {
+      "path": "../ui-slots"
+    },
     {
       "path": "../../../vendor/cordis"
     },