feat(sidepanel): 添加配置菜单，支持导入和导出设置功能

sidepanel/sidepanel.css

@@ -142,6 +142,67 @@ header {
 [data-theme="dark"] .theme-toggle .icon-moon { display: none; }
 [data-theme="dark"] .theme-toggle .icon-sun { display: block; }
 
+.header-menu {
+  position: relative;
+  display: flex;
+  align-items: center;
+}
+
+.header-config-btn {
+  min-width: 0;
+  padding-inline: 10px;
+}
+
+.header-config-btn[aria-expanded="true"] {
+  background: var(--bg-hover);
+  color: var(--text-primary);
+}
+
+.header-dropdown {
+  position: absolute;
+  top: calc(100% + 6px);
+  right: 0;
+  min-width: 120px;
+  padding: 6px;
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  background: var(--bg-base);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  box-shadow: var(--shadow-md);
+  z-index: 1300;
+}
+
+.header-dropdown[hidden] {
+  display: none !important;
+}
+
+.header-dropdown-item {
+  width: 100%;
+  padding: 8px 10px;
+  border: none;
+  border-radius: var(--radius-sm);
+  background: transparent;
+  color: var(--text-secondary);
+  font: inherit;
+  font-size: 12px;
+  font-weight: 600;
+  text-align: left;
+  cursor: pointer;
+  transition: background var(--transition), color var(--transition);
+}
+
+.header-dropdown-item:hover:not(:disabled) {
+  background: var(--bg-hover);
+  color: var(--text-primary);
+}
+
+.header-dropdown-item:disabled {
+  color: var(--text-muted);
+  cursor: not-allowed;
+}
+
 /* ============================================================
    Buttons
    ============================================================ */