feat(frontend): move settings into composer footer as live selectors

- Remove the microphone icon from the composer footer
- Left: workspace selector in its place
- Right: model + thinking-level selectors (persist via PUT /settings) next to
  the context-usage gauge
- Delete the Settings dialog (form + button); only "New session" remains

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/shell/AppShell.tsx

@@ -3,7 +3,6 @@ import { useSessionStore } from "../store/sessionStore";
 import { WidgetHost } from "./WidgetHost";
 import { Transcript } from "./Transcript";
 import { NavSessions } from "./NavSessions";
-import { SettingsDialog } from "./SettingsDialog";
 import { SteerInput, ComposerFooter } from "./SteerInput";
 import { WorkflowBar } from "./WorkflowBar";
 import { Button } from "../components/ui/button";
@@ -113,7 +112,6 @@ export function AppShell() {
           >
             New session
           </Button>
-          <SettingsDialog />
         </div>
 
         <div className="px-4 pb-1.5 pt-1">