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/shell/hooks/use-overlay-routing.ts

@@ -4,7 +4,7 @@ import { useLocation, useNavigate } from 'react-router-dom'
 import { type CommandCenterSection } from '@/app/command-center'
 import { AGENTS_ROUTE, appViewForPath, COMMAND_CENTER_ROUTE, NEW_CHAT_ROUTE } from '@/app/routes'
 
-const SECTIONS = ['models', 'sessions', 'system'] as const
+const SECTIONS = ['sessions', 'system', 'usage'] as const
 const OVERLAY_VIEWS = new Set(['settings', 'command-center', 'agents'])
 
 export function useOverlayRouting() {