feat(frontend): engine dropdown + emotion dual mode

- Replace engine buttons with <select> dropdown
- Add ENGINE_CONFIG for centralized engine management
- Add global/per-sentence emotion UI mode switching
- Add #engine-select dark theme styles
- Remove old .switch-btn and .engine-icon styles
- Reset emotion state on engine switch

frontend/app.js

@@ -3,10 +3,23 @@
  */
 
 const CONFIG = {
-    API_BASE_URL: '',  // 空字符串表示使用相对路径，通过Nginx代理
+    API_BASE_URL: '',
     MAX_CHARS: 2000,
 };
 
+const ENGINE_CONFIG = {
+    minimax: {
+        name: 'MiniMax',
+        emotionMode: 'global',
+        color: '#00d4aa'
+    },
+    cosyvoice: {
+        name: 'CosyVoice',
+        emotionMode: 'global',
+        color: '#ff6b6b'
+    }
+};
+
 // 情绪按钮与 API emotion 值的映射
 const EMOTION_MAP = {
     'happy': { emoji: '😊', label: '开心', apiValue: 'happy' },
@@ -30,8 +43,10 @@ const elements = {
     editor: document.getElementById('editor'),
     editorPlaceholder: document.getElementById('editorPlaceholder'),
     charCount: document.getElementById('charCount'),
-    switchBtns: document.querySelectorAll('.switch-btn'),
+    engineSelect: document.getElementById('engine-select'),
     emotionBtns: document.querySelectorAll('.emotion-btn'),
+    emotionGlobal: document.getElementById('emotion-global'),
+    emotionPerSentence: document.getElementById('emotion-per-sentence'),
     speedSlider: document.getElementById('speedSlider'),
     speedValue: document.getElementById('speedValue'),
     pitchSlider: document.getElementById('pitchSlider'),
@@ -143,12 +158,34 @@ function updateSliderDisplay() {
 
 function switchEngine(engine) {
     state.currentEngine = engine;
-    elements.switchBtns.forEach(btn => {
-        btn.classList.toggle('active', btn.dataset.engine === engine);
-    });
-    const engineName = engine === 'minimax' ? 'MiniMax' : 'CosyVoice';
-    document.getElementById('engineBadge').textContent = `蓝皓 · ${engineName}`;
-    showToast(`已切换到${engineName}引擎`);
+    const config = ENGINE_CONFIG[engine];
+    if (!config) return;
+
+    // Update engine badge
+    const badge = document.getElementById('engineBadge');
+    badge.textContent = `蓝皓 · ${config.name}`;
+    badge.style.background = config.color;
+
+    // Update engine select value
+    elements.engineSelect.value = engine;
+
+    // Switch emotion UI mode
+    updateEmotionMode(config.emotionMode);
+
+    // Reset emotion state
+    clearEmotionSelection();
+
+    showToast(`已切换到${config.name}引擎`);
+}
+
+function updateEmotionMode(mode) {
+    if (mode === 'global') {
+        elements.emotionGlobal.style.display = 'flex';
+        elements.emotionPerSentence.style.display = 'none';
+    } else if (mode === 'perSentence') {
+        elements.emotionGlobal.style.display = 'none';
+        elements.emotionPerSentence.style.display = 'block';
+    }
 }
 
 async function generateAudio() {
@@ -290,8 +327,8 @@ function downloadAudio() {
 
 elements.editor.addEventListener('input', updateCharCount);
 elements.editor.addEventListener('paste', () => { setTimeout(updateCharCount, 0); });
-elements.switchBtns.forEach(btn => {
-    btn.addEventListener('click', () => switchEngine(btn.dataset.engine));
+elements.engineSelect.addEventListener('change', (e) => {
+    switchEngine(e.target.value);
 });
 elements.emotionBtns.forEach(btn => {
     btn.addEventListener('click', () => {

frontend/index.html

@@ -37,17 +37,14 @@
                 <div class="toolbar">
                     <div class="toolbar-group">
                         <span class="toolbar-label">引擎：</span>
-                        <button class="switch-btn active" data-engine="minimax">
-                            <span class="engine-icon minimax-icon">M</span>
-                            MiniMax
-                        </button>
-                        <button class="switch-btn" data-engine="cosyvoice">
-                            <span class="engine-icon cosyvoice-icon">C</span>
-                            CosyVoice
-                        </button>
+                        <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-label">情绪：</span>
+                        <div id="emotion-global">
                             <button class="emotion-btn" data-emotion="happy" title="开心愉快">😊</button>
                             <button class="emotion-btn" data-emotion="sad" title="悲伤">😢</button>
                             <button class="emotion-btn" data-emotion="angry" title="激昂">🔥</button>
@@ -56,6 +53,10 @@
                             <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）
+                        </div>
+                    </div>
                 </div>
 
                 <!-- Editor -->

frontend/styles.css

@@ -121,30 +121,38 @@ body {
 .toolbar-label { font-size: 0.85rem; color: var(--text-secondary); }
 .toolbar-hint { font-size: 0.7rem; color: var(--text-muted); margin-left: 8px; }
 
-.switch-btn {
-    display: flex;
-    align-items: center;
-    gap: 4px;
-    padding: 4px 12px;
-    background: transparent;
-    border: 1px solid var(--border-color);
+#engine-select {
+    padding: 6px 12px;
+    background: #1a1a25;
+    border: 1px solid #2a2a3a;
     border-radius: var(--radius-sm);
-    color: var(--text-secondary);
+    color: #ffffff;
     font-size: 0.85rem;
     cursor: pointer;
     transition: var(--transition-fast);
+    outline: none;
+    -webkit-appearance: none;
+    appearance: none;
+    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='12' viewBox='0 0 12 12'%3E%3Cpath fill='%2371717a' d='M2 4l4 4 4-4'/%3E%3C/svg%3E");
+    background-repeat: no-repeat;
+    background-position: right 8px center;
+    padding-right: 28px;
 }
-.switch-btn:hover { border-color: var(--border-hover); color: var(--text-primary); }
-.switch-btn.active { background: var(--bg-card); border-color: var(--accent-primary); color: var(--text-primary); }
+#engine-select:hover { border-color: var(--border-hover); }
+#engine-select:focus { border-color: var(--accent-primary); }
+#engine-select option { background: #1a1a25; color: #ffffff; }
 
-.engine-icon {
-    width: 18px; height: 18px;
-    border-radius: var(--radius-sm);
-    display: flex; align-items: center; justify-content: center;
-    font-size: 0.65rem; font-weight: 700;
+#emotion-global {
+    display: flex;
+    align-items: center;
+    gap: 4px;
+}
+
+#emotion-per-sentence {
+    color: #71717a;
+    padding: 8px;
+    font-size: 0.85rem;
 }
-.minimax-icon { background: var(--minimax-color); color: #0a0a0f; }
-.cosyvoice-icon { background: var(--cosyvoice-color); color: white; }
 
 .emotion-btn {
     width: 32px; height: 32px;