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-models/README.md

@@ -0,0 +1,15 @@
+# @deepseek-ai/dsh-client-ui-models
+
+Models settings section plugin: registers the `models` nav entry into `settings.section` with an intentionally empty content column — model management lands in a later phase.
+
+## Model Experience
+
+None, as the section renders an empty browser UI column; nothing here reaches a model request.
+
+#### KV Cache effect
+
+None; this package neither assembles nor sends a provider request.
+
+## Known Limitations and Deferred Work
+
+- **Content column is empty by design** — provider list, editing form, and activation flow are deferred until the model-management service exists.