feat(settings): 用户名校验强化、个性签名编辑、字符计数器及提示优化

- 用户名校验改用白名单正则（中英文/数字/下划线/连字符），字符数计法统一
- 简介→个性签名，新增 textarea 可编辑，上限 128 字符
- 用户名+签名各增加实时字符计数器（x/16、x/128）
- API 合并为 PUT /api/settings/profile，一次提交两字段，按变更写入
- Toast 移至 NAV 下方，独立样式，5 秒自动消失
- 移除用户 ID 灰色底色，提示改为「个人资料已更新」
- 纯文本存储 + Go 模板自动转义防 XSS

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

@@ -10,6 +10,37 @@ body {
     min-height: 100vh;
 }
 
+/* ---------- Toast Notification ---------- */
+.settings-toast {
+    position: fixed;
+    top: 55px; /* 紧贴导航栏下方 */
+    left: 50%;
+    transform: translateX(-50%) translateY(-20px);
+    padding: .6rem 1.5rem;
+    font-size: .88rem;
+    font-weight: 500;
+    color: #fff;
+    background: #2ecc71;
+    border-radius: 8px;
+    box-shadow: 0 4px 16px rgba(0, 0, 0, .15);
+    z-index: 1000;
+    opacity: 0;
+    pointer-events: none;
+    transition: opacity .25s ease, transform .25s ease;
+    max-width: calc(100% - 40px);
+    white-space: nowrap;
+}
+
+.settings-toast.show {
+    opacity: 1;
+    transform: translateX(-50%) translateY(0);
+    pointer-events: auto;
+}
+
+.settings-toast.error {
+    background: #e74c3c;
+}
+
 /* ---------- Layout ---------- */
 .settings-layout {
     display: flex;
@@ -104,6 +135,91 @@ body {
     padding: 1.25rem 2rem 1.5rem;
 }
 
+/* ---------- Input Field ---------- */
+.settings-text-input {
+    width: 100%;
+    max-width: 260px;
+    padding: .45rem .7rem;
+    font-size: .92rem;
+    font-family: inherit;
+    color: var(--color-text);
+    background: #f9fafb;
+    border: 1px solid var(--color-border);
+    border-radius: 6px;
+    outline: none;
+    transition: border-color .15s ease, box-shadow .15s ease;
+}
+
+.settings-text-input:focus {
+    border-color: var(--color-accent);
+    box-shadow: 0 0 0 3px rgba(52, 152, 219, .15);
+    background: #fff;
+}
+
+/* ---------- Textarea Field ---------- */
+.settings-textarea {
+    width: 100%;
+    max-width: 460px;
+    height: 80px;
+    padding: .5rem .7rem;
+    font-size: .92rem;
+    font-family: inherit;
+    color: var(--color-text);
+    background: #f9fafb;
+    border: 1px solid var(--color-border);
+    border-radius: 6px;
+    outline: none;
+    resize: vertical;
+    transition: border-color .15s ease, box-shadow .15s ease;
+    line-height: 1.5;
+}
+
+.settings-textarea:focus {
+    border-color: var(--color-accent);
+    box-shadow: 0 0 0 3px rgba(52, 152, 219, .15);
+    background: #fff;
+}
+
+/* ---------- Character Counter ---------- */
+.char-counter {
+    font-size: .78rem;
+    color: #aaa;
+    margin-top: 4px;
+    display: block;
+}
+
+/* ---------- Save Button ---------- */
+.settings-card-footer {
+    padding: 1rem 2rem 1.5rem;
+    border-top: 1px solid var(--color-border);
+    margin-top: .25rem;
+}
+
+.settings-save-btn {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    padding: .55rem 1.5rem;
+    font-size: .88rem;
+    font-weight: 600;
+    font-family: inherit;
+    color: #fff;
+    background: var(--color-accent);
+    border: none;
+    border-radius: 6px;
+    cursor: pointer;
+    transition: background .15s ease, opacity .15s ease;
+}
+
+.settings-save-btn:hover {
+    background: #2980b9;
+}
+
+.settings-save-btn:disabled {
+    opacity: .6;
+    cursor: not-allowed;
+}
+
 /* ---------- Info Rows ---------- */
 .info-row {
     display: flex;