feat(frontend): pulse the stop dot while the harness is working

Give live feedback that something is happening the moment a session starts
processing: the red stop dot in the composer beats with a soft heartbeat + halo
while the harness is actively working (running — not waiting at a gate or
finalized), and settles the instant the model hands control back to the reviewer
or the session ends. Ring/scale only (no layout properties animated); honors
prefers-reduced-motion.

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

frontend/src/shell/AppShell.tsx

@@ -508,6 +508,7 @@ export function AppShell() {
                   onStop={() => setStopConfirm(true)}
                   inputRef={composerRef}
                   awaitingInput={awaitingQuestion || pendingWidget?.widget === "freetext"}
+                  working={running}
                 />
               </div>
               <ComposerFooter />