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/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;