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/tests/invariant.spec.ts

@@ -0,0 +1,23 @@
+import { describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import * as ModelsInvariant from '@deepseek-ai/dsh-client-ui-models/invariant'
+import InvariantService from '@deepseek-ai/dsh-invariants'
+import { ModelsSection } from '../src/client/ModelsSection.tsx'
+
+describe('invariant companion', () => {
+  it('registers under the package name with an empty installer', async () => {
+    const ctx = new Context()
+    await ctx.plugin(InvariantService, { enabled: true })
+    await expect(ctx.plugin(ModelsInvariant).await()).resolves.toBeDefined()
+  })
+
+  it('node-half apply is a no-op host placeholder', async () => {
+    const { apply } = await import('@deepseek-ai/dsh-client-ui-models')
+    apply()
+    expect(true).toBe(true) // reaching here without throw is the contract
+  })
+
+  it('the section content column is intentionally empty this phase', () => {
+    expect(ModelsSection()).toBeNull()
+  })
+})