fix(frontend): toolbar layout + engine-select styling

- Restructure toolbar to use toolbar-row for horizontal layout
- Fix emotion emoji buttons wrapping to vertical
- Enlarge engine-select with better padding, font-size, min-width
- Add toolbar-divider between engine and emotion sections
- Add hover/focus box-shadow to engine-select
- Remove toolbar-hint text to save space

frontend/index.html

@@ -35,14 +35,13 @@
 
                 <!-- Toolbar -->
                 <div class="toolbar">
-                    <div class="toolbar-group">
+                    <div class="toolbar-row">
                         <span class="toolbar-label">引擎：</span>
                         <select id="engine-select">
                             <option value="minimax">MiniMax</option>
                             <option value="cosyvoice">CosyVoice</option>
                         </select>
-                    </div>
-                    <div class="toolbar-group emotion-group">
+                        <span class="toolbar-divider"></span>
                         <span class="toolbar-label">情绪：</span>
                         <div id="emotion-global">
                             <button class="emotion-btn" data-emotion="happy" title="开心愉快">😊</button>
@@ -51,10 +50,9 @@
                             <button class="emotion-btn" data-emotion="surprised" title="惊讶">❗</button>
                             <button class="emotion-btn" data-emotion="fearful" title="害怕">😨</button>
                             <button class="emotion-btn" data-emotion="disgusted" title="厌恶">😒</button>
-                            <span class="toolbar-hint">← 选择整段语音的情绪风格</span>
                         </div>
                         <div id="emotion-per-sentence" style="display:none">
-                            逐句标注模式：选中句子后点击情绪标签（即将支持）
+                            逐句标注模式（即将支持）
                         </div>
                     </div>
                 </div>