feat(search): 点击放大镜切换搜索引擎

- 搜索框左侧图标改为触发器，点击展开效果五快捷栏
- 选中后左侧图标切换为对应引擎图标（本地放大镜 / Google / Bing / 百度 / 搜狗 / 360）
- 搜索框占位文案随当前模式变化，原有过滤与回车行为保留

stash-web/src/App.css

@@ -1444,42 +1444,37 @@ html.theme-transition .site-background {
   align-items: center;
 }
 
-.engine-toggle {
+.engine-trigger {
   display: inline-flex;
   align-items: center;
-  gap: 3px;
-  height: 24px;
-  padding: 0 var(--su-space-2);
-  border: 1px solid var(--su-border);
+  justify-content: center;
+  width: 26px;
+  height: 26px;
+  padding: 0;
+  border: 0;
   border-radius: var(--su-radius-xs);
-  background-color: var(--su-bg-subtle);
-  color: var(--su-text-sub);
-  font: inherit;
-  font-size: 11px;
-  line-height: 1;
+  background: transparent;
+  color: var(--su-text-muted);
   cursor: pointer;
   transition:
     color var(--su-duration-fast) var(--su-ease-in-out),
-    background-color var(--su-duration-fast) var(--su-ease-in-out),
-    border-color var(--su-duration-fast) var(--su-ease-in-out);
+    background-color var(--su-duration-fast) var(--su-ease-in-out);
 }
 
-.engine-toggle:hover,
-.engine-toggle[aria-expanded='true'] {
-  border-color: var(--su-border-strong);
-  background-color: var(--su-bg-hover);
+.engine-trigger:hover,
+.engine-trigger[aria-expanded='true'] {
+  background-color: var(--su-bg-subtle);
   color: var(--su-text-main);
 }
 
-.engine-toggle span {
-  font-size: 9px;
-  opacity: 0.6;
+.engine-trigger svg {
+  display: block;
 }
 
 .engine-menu {
   position: absolute;
   top: 50%;
-  right: calc(100% + 6px);
+  left: calc(100% + 6px);
   z-index: var(--su-z-float);
   display: flex;
   flex-direction: row;
@@ -1496,7 +1491,7 @@ html.theme-transition .site-background {
 @keyframes engine-bar-in {
   from {
     opacity: 0;
-    transform: translate(6px, -50%);
+    transform: translate(-6px, -50%);
   }
   to {
     opacity: 1;