feat: 扩展工作空间侧边栏宽度约束并统一拖动手柄样式

- 侧边栏最大宽度从 800px 扩展至 1200px（含动态计算）
- 文件树最小宽度从 180px 提升至 300px
- 文件树最大宽度扩展为侧边栏宽度
- 预览关闭时文件树自动占满侧边栏，分隔线隐藏
- 外部拖动手柄改为与内部分隔线一致的样式（1px、hover 变色）

src/styles/components/workspace/_index.scss

@@ -6,7 +6,7 @@
 .workspace-sidebar {
     width: 500px;
     min-width: 400px;
-    max-width: 800px;
+    max-width: 1200px;
     background: var(--color-bg-2);
     border-left: 1px solid var(--color-border-2);
     display: flex;
@@ -131,8 +131,8 @@
 
     // ========== FileTree ==========
     &__file-tree {
-        min-width: 180px;
-        max-width: 300px;
+        min-width: 300px;
+        max-width: 100%;
         flex-shrink: 0;
         overflow-y: auto;
         padding: 8px;
@@ -177,39 +177,28 @@
     &__resize-handle {
         position: absolute;
         left: 0;
-        top: 50%;
-        transform: translateY(-50%);
-        width: 12px;
-        height: 48px;
-        display: flex;
-        align-items: center;
-        justify-content: center;
+        top: 0;
+        bottom: 0;
+        width: 1px;
+        background: var(--color-border-2);
         cursor: ew-resize;
-        opacity: 0.5;
-        transition: opacity 0.2s;
+        transition: background 0.2s;
         z-index: 10;
 
+        &::before {
+            content: '';
+            position: absolute;
+            top: 0;
+            left: -4px;
+            right: -4px;
+            bottom: 0;
+            cursor: ew-resize;
+        }
+
         &:hover {
-            opacity: 1;
+            background: var(--color-primary);
         }
     }
-
-    &__resize-lines {
-        display: flex;
-        flex-direction: column;
-        gap: 2px;
-
-        div {
-            width: 4px;
-            height: 2px;
-            background: var(--color-text-3);
-            border-radius: 1px;
-        }
-    }
-
-    &__resize-handle:hover &__resize-lines div {
-        background: var(--color-text-1);
-    }
 }
 
 // 工作空间展开按钮