agent ui: reset cancel-turn busy state via .finally(), not a thinking-flip effect

argus (PR review): the thinking-flip useEffect left the button stuck
disabled if the /api/cancel POST itself failed while the turn was
still genuinely thinking, since nothing re-fires the effect. Switch
onCancelTurn's signature to () => Promise<void> and reset cancelBusy
in a .finally() on that promise instead — fires on success or
failure alike, matching app.js's original
postCancelTurn().finally(() => { btn.disabled = false; }).

frontend/packages/agent/src/Root.tsx

@@ -155,7 +155,7 @@ export function Root() {
             paused={false}
             onTogglePause={() => {}}
             thinking={false}
-            onCancelTurn={() => {}}
+            onCancelTurn={() => Promise.resolve()}
           />
         </Header>
         <main className="agent-main">
@@ -210,12 +210,12 @@ export function Root() {
             );
           }}
           thinking={effectiveTurnState === 'thinking'}
-          onCancelTurn={() => {
+          onCancelTurn={() =>
             postCancelTurn().then((r) => {
               if (!r.ok) liveStreamRef.current?.pushNote(`✗ /cancel failed${r.detail ? ': ' + r.detail : ''}`);
               refresh();
-            });
-          }}
+            })
+          }
         />
       </Header>
       <main className="agent-main">