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', () => {