refactor(desktop): move model management from Command Center into Settings

Command Center's Models section and Settings > Model rendered the same
model state with identical persistence semantics — both write config and
apply to new sessions only (POST /api/model/set). The Command Center UI
was strictly better (provider catalog, curated model lists, friendly
auxiliary-task labels, Nous-gateway auto-routing on main-provider switch),
while Settings > Model was three barebones config fields.

Extract that UI into a shared settings/model-settings.tsx (restyled with
Settings primitives) and render it at the top of Settings > Model: main
model picker via setModelAssignment + the 9 auxiliary task slots with
per-task set-to-main / change / reset-all. model_context_length and
fallback_providers stay as config fields below it; the raw auxiliary.*
keys are dropped from Advanced (now covered by the panel).

Strip the Models section from Command Center entirely (section, state,
handlers, render, nav, search entry) leaving it focused on Sessions /
System / Usage, and move the live store-sync callback (onMainModelChanged)
from CommandCenterView to SettingsView. The composer's per-session model
picker (the only live hot-swap, via /model) is unchanged.

apps/desktop/src/app/desktop-controller.tsx

@@ -531,6 +531,13 @@ export function DesktopController() {
               void refreshCurrentModel()
               void queryClient.invalidateQueries({ queryKey: ['model-options'] })
             }}
+            onMainModelChanged={(provider, model) => {
+              setCurrentProvider(provider)
+              setCurrentModel(model)
+              updateModelOptionsCache(provider, model, true)
+              void refreshCurrentModel()
+              void queryClient.invalidateQueries({ queryKey: ['model-options'] })
+            }}
           />
         </Suspense>
       )}
@@ -541,13 +548,6 @@ export function DesktopController() {
             initialSection={commandCenterInitialSection}
             onClose={closeOverlayToPreviousRoute}
             onDeleteSession={removeSession}
-            onMainModelChanged={(provider, model) => {
-              setCurrentProvider(provider)
-              setCurrentModel(model)
-              updateModelOptionsCache(provider, model, true)
-              void refreshCurrentModel()
-              void queryClient.invalidateQueries({ queryKey: ['model-options'] })
-            }}
             onNavigateRoute={path => navigate(path)}
             onOpenSession={sessionId => navigate(sessionRoute(sessionId))}
           />