refactor: 主题选择改为调色盘图标+下拉菜单

- 🎨 图标按钮点击弹出下拉菜单
- 4 个中文主题选项：赤红合成/霓虹青蓝/琥珀终端/紫雾迷离
- 每项带彩色色块指示
- 当前主题高亮显示
- 点击外部自动关闭下拉

style.css

@@ -202,33 +202,86 @@
     font-weight: 400;
 }
 
-.bme-theme-palette {
+.bme-theme-picker {
+    position: relative;
+    margin-right: 4px;
+}
+
+.bme-theme-picker-btn {
+    width: 28px;
+    height: 28px;
     display: flex;
     align-items: center;
-    gap: 6px;
-    margin-right: 8px;
-}
-
-.bme-theme-dot {
-    width: 16px;
-    height: 16px;
-    border-radius: 50%;
-    border: 2px solid transparent;
-    background: var(--dot-color);
+    justify-content: center;
+    border: none;
+    background: transparent;
+    color: var(--bme-on-surface-dim);
     cursor: pointer;
-    padding: 0;
-    transition: transform 0.15s, border-color 0.15s, box-shadow 0.15s;
-    box-shadow: 0 0 0 0 transparent;
+    border-radius: 4px;
+    font-size: 14px;
+    transition: all 0.15s;
 }
 
-.bme-theme-dot:hover {
-    transform: scale(1.2);
-    box-shadow: 0 0 6px var(--dot-color);
+.bme-theme-picker-btn:hover {
+    background: var(--bme-surface-highest);
+    color: var(--bme-primary);
 }
 
-.bme-theme-dot.active {
-    border-color: var(--bme-on-surface, #e4e1e6);
-    box-shadow: 0 0 8px var(--dot-color);
+.bme-theme-dropdown {
+    display: none;
+    position: absolute;
+    top: 100%;
+    right: 0;
+    margin-top: 6px;
+    background: var(--bme-surface-container, #1f1f22);
+    border: 1px solid var(--bme-border);
+    border-radius: 8px;
+    padding: 4px;
+    min-width: 130px;
+    box-shadow: 0 8px 24px rgba(0, 0, 0, 0.5);
+    z-index: 100;
+    animation: bme-dropdown-in 0.12s ease-out;
+}
+
+.bme-theme-dropdown.open {
+    display: block;
+}
+
+@keyframes bme-dropdown-in {
+    from { opacity: 0; transform: translateY(-4px); }
+    to   { opacity: 1; transform: translateY(0); }
+}
+
+.bme-theme-option {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    width: 100%;
+    padding: 7px 10px;
+    border: none;
+    background: transparent;
+    color: var(--bme-on-surface);
+    font-size: 12.5px;
+    cursor: pointer;
+    border-radius: 5px;
+    transition: background 0.12s;
+    white-space: nowrap;
+}
+
+.bme-theme-option:hover {
+    background: var(--bme-surface-high);
+}
+
+.bme-theme-option.active {
+    background: var(--bme-primary-dim);
+    color: var(--bme-primary-text);
+}
+
+.bme-theme-swatch {
+    width: 10px;
+    height: 10px;
+    border-radius: 50%;
+    flex-shrink: 0;
 }
 
 .bme-panel-close {