feat(frontend): rework chat composer (sticky input, inline new session, status footer)

- Center ThothII logo + tagline; break "Human In The Loop" onto a second line
- Rename "New question" → "New session"; the button now clears the screen and
  focuses the sticky bottom composer instead of opening a dialog
- Composer is always rendered and pinned at the bottom; typing the first
  message there creates the session (no modal)
- Add inline stop control (square + inner circle) that interrupts and saves
  the session via /close
- Always-present send affordance (CornerDownLeft ↵) on the right of the box
- Status footer under the box: mic (inert), model name, thinking level, and a
  context-usage gauge (placeholder)
- Brand-tinted spinner beside the logo while the harness/backend holds the
  ball; hidden once a widget needs human input

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

frontend/src/shell/NavSessions.tsx

@@ -26,7 +26,7 @@ export function NavSessions({ activeSessionId, onSessionSelect }: Props) {
   if (sessions.length === 0) {
     return (
       <p className="px-1 py-6 text-center text-xs text-muted-foreground">
-        No sessions yet. Start with <span className="font-medium text-foreground">New question</span>.
+        No sessions yet. Start with <span className="font-medium text-foreground">New session</span>.
       </p>
     );
   }