feat: @提及功能优化 + 默认头像改用SVG图标

- 评论区有效@提及渲染为蓝色链接跳转/space/{uid}，无效@保持纯文本
- 输入@触发悬浮窗，根据输入实时搜索用户（含头像+等级）
- 无匹配时显示未搜索到用户，选中后插入为@用户名 格式（尾部空格）
- 悬浮窗定位于输入光标上方，不遮挡输入内容
- 后端Comment新增Mentions字段，批量加载提及映射供前端判断
- SearchUsers/parseMentions 统一限制LV2+用户
- 所有默认头像由首字符文字改为统一SVG人形图标（nav/home/space/mention等7处）

templates/MetaLab-2026/static/css/common.css

@@ -230,11 +230,13 @@ header {
     align-items: center;
     justify-content: center;
     background: var(--color-accent);
-    color: #fff;
-    font-size: .85rem;
-    font-weight: 600;
+    color: rgba(255,255,255,.75);
     object-fit: unset;
 }
+.nav-avatar-default svg {
+    width: 55%;
+    height: 55%;
+}
 
 .nav-avatar-wrapper:hover .nav-avatar {
     border-color: var(--color-accent);
@@ -279,9 +281,11 @@ header {
     display: flex;
     align-items: center;
     justify-content: center;
-    color: #fff;
-    font-size: 1.1rem;
-    font-weight: 600;
+    color: rgba(255,255,255,.75);
+}
+.nav-dropdown-avatar svg {
+    width: 52%;
+    height: 52%;
 }
 
 .nav-dropdown-avatar img {

templates/MetaLab-2026/static/css/home.css

@@ -239,14 +239,15 @@
     height: 64px;
     border-radius: 50%;
     background: linear-gradient(135deg, var(--color-accent), #2980b9);
-    color: #fff;
+    color: rgba(255,255,255,.75);
     display: flex;
     align-items: center;
     justify-content: center;
     margin: 0 auto .75rem;
-    font-size: 1.6rem;
-    font-weight: 700;
-    text-transform: uppercase;
+}
+.user-avatar-fallback svg {
+    width: 55%;
+    height: 55%;
 }
 
 .guest-avatar {

templates/MetaLab-2026/static/css/posts.css

@@ -1127,6 +1127,18 @@
     margin-right: 6px;
 }
 
+/* 有效@提及链接 */
+.mention-link {
+    color: #6366f1;
+    text-decoration: none;
+    font-weight: 500;
+}
+
+.mention-link:hover {
+    text-decoration: underline;
+    color: #4f46e5;
+}
+
 .comment-actions {
     display: flex;
     gap: 16px;
@@ -1238,6 +1250,9 @@
 }
 
 .mention-item {
+    display: flex;
+    align-items: center;
+    gap: 8px;
     padding: 8px 14px;
     font-size: 13px;
     cursor: pointer;
@@ -1250,14 +1265,47 @@
     color: #6366f1;
 }
 
-.mention-item .mention-username {
-    font-weight: 500;
+.mention-item-avatar {
+    width: 24px;
+    height: 24px;
+    border-radius: 50%;
+    object-fit: cover;
+    flex-shrink: 0;
+    background: #f3f4f6;
 }
 
-.mention-item .mention-uid {
+.mention-item-avatar-placeholder {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    width: 24px;
+    height: 24px;
+    border-radius: 50%;
+    background: #e5e7eb;
+    flex-shrink: 0;
+    color: #9ca3af;
+}
+.mention-item-avatar-placeholder svg {
+    width: 60%;
+    height: 60%;
+}
+
+.mention-item .mention-username {
+    font-weight: 500;
+    flex: 1;
+}
+
+.mention-item .mention-level {
     color: #9ca3af;
     font-size: 11px;
-    margin-left: 8px;
+    flex-shrink: 0;
+}
+
+.mention-no-result {
+    padding: 12px 14px;
+    font-size: 13px;
+    color: #9ca3af;
+    text-align: center;
 }
 
 /* ========== 右下角悬浮操作栏 ========== */

templates/MetaLab-2026/static/css/space.css

@@ -75,14 +75,16 @@
     width: 56px; height: 56px;
     border-radius: 50%;
     background: rgba(255,255,255,.25);
-    color: #fff;
+    color: rgba(255,255,255,.75);
     display: flex;
     align-items: center;
     justify-content: center;
-    font-size: 1.4rem;
-    font-weight: 700;
     border: 2px solid rgba(255,255,255,.4);
 }
+.space-avatar-placeholder svg {
+    width: 55%;
+    height: 55%;
+}
 
 .space-header-info {
     color: #fff;
@@ -377,9 +379,12 @@
 }
 .space-user-avatar-placeholder {
     width: 48px; height: 48px; border-radius: 50%;
-    background: var(--color-accent); color: #fff;
+    background: var(--color-accent); color: rgba(255,255,255,.75);
     display: flex; align-items: center; justify-content: center;
-    font-size: 1.15rem; font-weight: 700;
+}
+.space-user-avatar-placeholder svg {
+    width: 55%;
+    height: 55%;
 }
 
 .space-user-info { flex: 1; min-width: 0; }