侧边栏图标统一为线条SVG + hover品牌色过渡

- 新增 Icon 组件:8种线条风格 SVG 图标(collection/chart/target/repeat等)
- 统一 stroke-width:1.5,hover 加粗到 2
- hover 时颜色平滑过渡到品牌色(press red)
- 添加 .nav-icon 和 .nav-icon-wrap CSS 类
- 活跃/悬停态:opacity 0.5→1,stroke-width 1.5→2

Co-Authored-By: new-provider/LongCat-2.0 <<EMAIL>>

frontend/src/index.css

@@ -157,6 +157,22 @@
   .skeleton {
     @apply animate-pulse rounded-none bg-ink-200;
   }
+
+  /* ── 侧边栏图标:统一样式 + hover过渡到品牌色 ── */
+  .nav-icon {
+    width: 1.125rem;
+    height: 1.125rem;
+    stroke-width: 1.5;
+    flex-shrink: 0;
+    opacity: 0.5;
+    transition: all 0.2s ease;
+  }
+  .nav-icon-wrap:hover .nav-icon,
+  .nav-icon-wrap.active .nav-icon {
+    opacity: 1;
+    color: #9E1B1B;
+    stroke-width: 2;
+  }
 }
 
 /* ── Admin 后台 ──