feat(home): add homepage hero banner and migrate community stats

- 新增首页欢迎横幅组件，包含时段问候、用户问候语、发帖引导按钮和社区统计数据
- 从CommunityPanels中移除stats相关逻辑，将社区统计迁移到新的HomeHero组件中
- 优化全局样式，添加横幅相关的CSS动画和样式类

frontend/app/globals.css

@@ -582,6 +582,77 @@ h1, h2, h3, h4 {
     border-color: color-mix(in srgb, var(--accent) 32%, var(--line));
   }
 
+  /* 首页欢迎横幅 */
+  .j13-hero {
+    position: relative;
+    overflow: hidden;
+    padding: 20px 22px;
+    border-radius: var(--r);
+    border: 1px solid var(--line);
+    box-shadow: var(--shadow-sm);
+    background:
+      linear-gradient(115deg, color-mix(in srgb, var(--accent-soft) 75%, var(--panel)) 0%, var(--panel) 60%);
+  }
+  .j13-hero-glow {
+    position: absolute;
+    top: -72px;
+    right: -52px;
+    width: 200px;
+    height: 200px;
+    border-radius: 9999px;
+    background: var(--accent-soft);
+    pointer-events: none;
+  }
+  .j13-hero-stats {
+    display: grid;
+    grid-template-columns: repeat(4, minmax(0, 1fr));
+    gap: 12px 10px;
+    width: 100%;
+  }
+  .j13-hero-stat {
+    display: flex;
+    flex-direction: column;
+    gap: 3px;
+    min-width: 0;
+  }
+  .j13-hero-stat-num {
+    font-size: 21px;
+    font-weight: 800;
+    line-height: 1.15;
+    letter-spacing: -0.015em;
+    color: var(--ink);
+    font-variant-numeric: tabular-nums;
+  }
+  .j13-hero-stat-num.is-hot { color: var(--accent); }
+  .j13-hero-stat-lab {
+    display: inline-flex;
+    align-items: center;
+    gap: 4px;
+    font-size: 12px;
+    color: var(--ink-3);
+    white-space: nowrap;
+  }
+  .j13-dot-pulse {
+    width: 6px;
+    height: 6px;
+    border-radius: 9999px;
+    background: var(--accent);
+    animation: j13-dot-pulse 2.4s ease-in-out infinite;
+  }
+  @media (min-width: 1024px) {
+    .j13-hero { padding: 22px 26px; }
+    .j13-hero-stats {
+      display: flex;
+      width: auto;
+      gap: 0;
+    }
+    .j13-hero-stat + .j13-hero-stat {
+      margin-left: 22px;
+      padding-left: 22px;
+      border-left: 1px solid var(--line);
+    }
+  }
+
   /* 移动端横向板块条（滚动条隐藏，保留触控惯性） */
   .j13-chips {
     display: flex;
@@ -739,6 +810,11 @@ h1, h2, h3, h4 {
 }
 .j13-fade-up { animation: j13-fade-up 0.2s ease-out both; }
 
+@keyframes j13-dot-pulse {
+  0%, 100% { opacity: 1; transform: scale(1); }
+  50% { opacity: 0.35; transform: scale(0.8); }
+}
+
 /* 跳到主内容 */
 .skip-link {
   position: fixed;