feat: SSR 私信与通知（/messages）

会话列表、读写、未读角标与全部已读；系统会话只读；用户主页可发起私信。

Co-authored-by: Cursor <cursoragent@cursor.com>

public/assets/site.css

@@ -378,5 +378,59 @@ body.j13-body {
 .j13-user-hero h1 { margin: 0 0 0.35rem; font-size: 1.35rem; }
 .j13-user-sig { margin: 0.5rem 0 0; white-space: pre-wrap; }
 
+.j13-badge {
+  display: inline-block;
+  min-width: 1.1rem;
+  padding: 0.05rem 0.35rem;
+  border-radius: 999px;
+  background: var(--j13-accent);
+  color: #fff;
+  font-size: 0.7rem;
+  font-weight: 600;
+  line-height: 1.2;
+  text-align: center;
+  vertical-align: middle;
+}
+.j13-messages { max-width: 720px; margin: 0 auto; padding: 1rem 1rem 2.5rem; }
+.j13-messages h1 { margin: 0 0 0.75rem; font-size: 1.35rem; }
+.j13-msg-list { list-style: none; margin: 0; padding: 0; }
+.j13-msg-item {
+  border-bottom: 1px solid var(--j13-border);
+}
+.j13-msg-item a {
+  display: grid;
+  gap: 0.2rem;
+  padding: 0.85rem 0;
+  color: inherit;
+  text-decoration: none;
+}
+.j13-msg-item.is-unread a strong { color: var(--j13-accent); }
+.j13-msg-item__preview {
+  color: var(--j13-muted);
+  font-size: 0.9rem;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+.j13-msg-thread { list-style: none; margin: 1rem 0; padding: 0; display: flex; flex-direction: column; gap: 0.75rem; }
+.j13-msg-bubble {
+  max-width: 92%;
+  padding: 0.65rem 0.85rem;
+  border: 1px solid var(--j13-border);
+  border-radius: var(--j13-radius);
+  background: var(--j13-surface);
+  align-self: flex-start;
+}
+.j13-msg-bubble.is-self {
+  align-self: flex-end;
+  background: #eff6ff;
+  border-color: #bfdbfe;
+}
+.j13-msg-bubble.is-system { max-width: 100%; }
+.j13-msg-bubble__subj { font-weight: 600; margin-bottom: 0.25rem; }
+.j13-msg-bubble__body { white-space: pre-wrap; overflow-wrap: anywhere; }
+.j13-msg-compose { margin-top: 1.25rem; }
+
+