fix: 隐藏观察模式按钮，清理未使用的 toggleMode 引用

frontend/src/App.tsx

@@ -69,7 +69,6 @@ function AppContent() {
     stats,
     mode,
     isObserving,
-    toggleMode,
     startSession,
     stopSession,
     interrupt,
@@ -292,12 +291,6 @@ function AppContent() {
                 >
                   🎤
                 </button>
-                <button
-                  className={`btn ${mode === "observation" ? "btn--active" : "btn--secondary"}`}
-                  onClick={toggleMode}
-                >
-                  {mode === "observation" ? tr("controls.observing") : tr("controls.observation")}
-                </button>
                 {isProcessing && (
                   <button className="btn btn--warning" onClick={interrupt}>
                     {tr("controls.interrupt")}