fix(admin/workbench): stat-card 改单色 var(--brand-primary), 跟 manager/Workbench 一致

上次 0f124de commit 用了 linear-gradient + 蓝色阴影, 跟项目其它 workbench 不一致.

按用户要求:
- 背景: linear-gradient → var(--brand-primary) 单色 (不用渐变)
- 边框: 删 → 加 1px solid var(--brand-primary) (单色, 跟 manager 同款)
- hover: translateY + box-shadow → opacity .9 + translateY(-2px) (manager 同款)
- label 颜色: rgba(255,255,255,0.75) → rgba(255,255,255,0.85) (跟 manager 一致)

现在 admin/manager 两个 workbench 的 stat-card 视觉完全一致, 都用系统主色变量.

ry-vue3/src/views/admin/Workbench.vue

@@ -99,7 +99,8 @@ onMounted(load)
 
 .stats-grid { display: grid; grid-template-columns: repeat(5, 1fr); gap: 16px; margin-bottom: 12px; }
 .stat-card {
-  background: linear-gradient(135deg, var(--brand-primary) 0%, var(--brand-primary-deep) 100%);
+  background: var(--brand-primary);
+  border: 1px solid var(--brand-primary);
   border-radius: 8px;
   padding: 24px;
   cursor: pointer;
@@ -109,11 +110,11 @@ onMounted(load)
   flex-direction: column;
   align-items: center;
   justify-content: center;
-  transition: transform 0.2s, box-shadow 0.2s;
+  transition: opacity .15s, transform .15s;
   min-height: 120px;
 }
-.stat-card:hover { transform: translateY(-2px); box-shadow: 0 6px 16px rgba(21,42,102,0.4); }
-.stat-label { font-size: 14px; color: rgba(255,255,255,0.75); margin-bottom: 16px; }
+.stat-card:hover { opacity: .9; transform: translateY(-2px); }
+.stat-label { font-size: 14px; color: rgba(255, 255, 255, 0.85); margin-bottom: 16px; }
 .stat-value {
   font-size: 32px;
   font-weight: 600;