frontend: move the second-level menu to the left of the content

The rail was on the right. It now leads the row instead, so the section menu
reads as a conventional left-hand navigation column.

Which side it lands on was never a stylesheet decision: `.app-body` is a flex
row and the aside is simply its first child now, with `.app-main` after it.
The only style change that had to follow is the rail's separator, which moves
from its left edge to its right, plus the collapse tooltip's placement.

The global shell contract in base.css now says this explicitly, so the next
person to move the rail knows the side comes from DOM order rather than from a
rule in that file.

frontend/src/components/SectionAside.vue

@@ -1,6 +1,6 @@
 <script setup lang="ts">
 /**
- * The second-level menu, pinned to the right of the content area.
+ * The second-level menu, pinned to the left of the content area.
  *
  * It is driven entirely by `route.meta.section`, so the menu a URL belongs to
  * is a property of the route table rather than of component state — deep
@@ -58,7 +58,7 @@ const collapsed = computed(() => appStore.asideCollapsed)
       <span v-if="!collapsed" class="aside-title">{{ menu.title }}</span>
       <el-tooltip
         :content="collapsed ? '展开菜单' : '收起菜单'"
-        placement="left"
+        placement="right"
         :show-after="200"
       >
         <el-button
@@ -94,8 +94,9 @@ const collapsed = computed(() => appStore.asideCollapsed)
 .app-aside {
   display: flex;
   flex-direction: column;
-  /* The wrapper owns the border so it stays put while the menu scrolls. */
-  border-left: 1px solid var(--el-border-color);
+  /* The wrapper owns the border so it stays put while the menu scrolls. The
+     rail sits at the left edge, so the separator goes on its right. */
+  border-right: 1px solid var(--el-border-color);
   background-color: var(--el-bg-color);
   overflow: hidden;
 }