feat: 重构侧栏布局，优化用户菜单和同步功能

frontend/src/style.css

@@ -161,36 +161,6 @@ label:not(.semi-checkbox):not(.semi-radio) {
   flex-direction: column;
   overflow: hidden;
 }
-.library-stats {
-  display: flex;
-  align-items: center;
-  flex-shrink: 0;
-  gap: var(--space-4);
-  padding: 0 0 var(--space-3);
-  flex-wrap: wrap;
-}
-.library-stats > div {
-  display: flex;
-  align-items: baseline;
-  gap: var(--space-2);
-}
-.library-stats span {
-  font-size: 12px;
-  color: var(--muted);
-}
-.library-stats strong {
-  font-size: 16px;
-  font-weight: 500;
-  font-variant-numeric: tabular-nums;
-}
-.library-stats .last-sync {
-  margin-left: auto;
-}
-.library-stats .last-sync strong {
-  font-size: 12px;
-  font-weight: 400;
-  color: var(--muted);
-}
 .library-panel {
   display: flex;
   flex-direction: column;
@@ -210,9 +180,28 @@ label:not(.semi-checkbox):not(.semi-radio) {
   max-height: 44%;
   overflow: auto;
 }
-.alpha-submission-tabs {
+.alpha-tabs-header {
+  display: flex;
+  align-items: center;
+  gap: var(--space-2);
   flex-shrink: 0;
   padding: 0 var(--space-3);
+  border-bottom: 1px solid var(--line);
+}
+.alpha-submission-tabs {
+  min-width: 0;
+  flex-shrink: 0;
+}
+.alpha-last-sync {
+  margin-left: auto;
+  padding: var(--space-2) 0;
+  color: var(--muted);
+  font-size: 12px;
+  text-align: right;
+}
+.alpha-last-sync > span {
+  display: inline-block;
+  font-variant-numeric: tabular-nums;
 }
 .alpha-submission-tabs .semi-tabs-content {
   display: none;
@@ -782,9 +771,6 @@ label:not(.semi-checkbox):not(.semi-radio) {
   .advanced-grid {
     grid-template-columns: repeat(3, minmax(0, 1fr));
   }
-  .library-stats .last-sync {
-    display: none;
-  }
   .preferences-grid {
     grid-template-columns: 1fr 1fr;
   }
@@ -804,9 +790,6 @@ label:not(.semi-checkbox):not(.semi-radio) {
     align-items: flex-start;
     flex-wrap: wrap;
   }
-  .library-stats {
-    gap: var(--space-3);
-  }
   .primary-filters {
     display: grid;
     grid-template-columns: 1fr 1fr;