feat：样式美化

frontend/src/App.tsx

@@ -108,8 +108,12 @@ function App() {
             className="btn-icon"
             onClick={() => setShowConfig((v) => !v)}
             title="设置"
+            aria-label="设置"
           >
-            ⚙️
+            <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="1.8" strokeLinecap="round" strokeLinejoin="round">
+              <circle cx="12" cy="12" r="3" />
+              <path d="M19.4 15a1.65 1.65 0 0 0 .33 1.82l.06.06a2 2 0 0 1-2.83 2.83l-.06-.06a1.65 1.65 0 0 0-1.82-.33 1.65 1.65 0 0 0-1 1.51V21a2 2 0 0 1-4 0v-.09A1.65 1.65 0 0 0 9 19.4a1.65 1.65 0 0 0-1.82.33l-.06.06a2 2 0 0 1-2.83-2.83l.06-.06A1.65 1.65 0 0 0 4.68 15a1.65 1.65 0 0 0-1.51-1H3a2 2 0 0 1 0-4h.09A1.65 1.65 0 0 0 4.6 9a1.65 1.65 0 0 0-.33-1.82l-.06-.06a2 2 0 0 1 2.83-2.83l.06.06A1.65 1.65 0 0 0 9 4.68a1.65 1.65 0 0 0 1-1.51V3a2 2 0 0 1 4 0v.09a1.65 1.65 0 0 0 1 1.51 1.65 1.65 0 0 0 1.82-.33l.06-.06a2 2 0 0 1 2.83 2.83l-.06.06A1.65 1.65 0 0 0 19.4 9a1.65 1.65 0 0 0 1.51 1H21a2 2 0 0 1 0 4h-.09a1.65 1.65 0 0 0-1.51 1z" />
+            </svg>
           </button>
         </div>
       </header>
@@ -161,8 +165,11 @@ function App() {
             )}
             {!isConnected && !stream && (
               <div className="video-placeholder">
-                <span className="video-placeholder__icon">📷</span>
-                <span>点击右侧按钮开始对话</span>
+                <svg className="video-placeholder__icon" width="48" height="48" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="1.2" strokeLinecap="round" strokeLinejoin="round">
+                  <path d="M23 19a2 2 0 0 1-2 2H3a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h4l2-3h6l2 3h4a2 2 0 0 1 2 2z" />
+                  <circle cx="12" cy="13" r="4" />
+                </svg>
+                <span>点击下方按钮开始对话</span>
               </div>
             )}
           </div>