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>

templates/messages/list.tmpl

@@ -0,0 +1,40 @@
+{{define "messages/list"}}
+{{template "base/head" .}}
+{{template "base/navbar" .}}
+<main class="j13-main j13-messages">
+  <h1>私信与通知</h1>
+  {{template "base/alert" .}}
+  <p class="j13-muted">
+    未读 {{.UnreadTotal}}
+    （私信 {{.UnreadDM}} · 通知 {{.UnreadNotify}}）
+    {{if gt .UnreadTotal 0}}
+    <form method="post" action="/messages/read-all" class="j13-inline-form">
+      <input type="hidden" name="_csrf" value="{{.CSRF}}"/>
+      <button type="submit" class="j13-linkbtn">全部标已读</button>
+    </form>
+    {{end}}
+  </p>
+
+  {{if .Conversations}}
+  <ul class="j13-msg-list">
+    {{range .Conversations}}
+    <li class="j13-msg-item{{if gt .Unread 0}} is-unread{{end}}">
+      <a href="/messages/with/{{.PeerID}}">
+        <strong>{{.PeerName}}</strong>
+        {{if gt .Unread 0}}<span class="j13-badge">{{.Unread}}</span>{{end}}
+        <span class="j13-msg-item__preview">{{.Preview}}</span>
+        <span class="j13-muted">{{.UpdatedLabel}}</span>
+      </a>
+    </li>
+    {{end}}
+  </ul>
+  <nav class="j13-pager">
+    {{if .HasPrev}}<a href="/messages?page={{.PrevPage}}">上一页</a>{{end}}
+    {{if .HasMore}}<a href="/messages?page={{.NextPage}}">下一页</a>{{end}}
+  </nav>
+  {{else}}
+  <p class="j13-empty">暂无会话。可在用户主页点击「发私信」开始对话。</p>
+  {{end}}
+</main>
+{{template "base/footer" .}}
+{{end}}

templates/messages/thread.tmpl

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+{{define "messages/thread"}}
+{{template "base/head" .}}
+{{template "base/navbar" .}}
+<main class="j13-main j13-messages">
+  <p><a href="/messages">← 返回会话列表</a></p>
+  <h1>{{.PeerName}}</h1>
+  {{if .IsSystem}}<p class="j13-muted">系统通知（只读）</p>{{else}}<p class="j13-muted"><a href="/user/{{.PeerID}}">查看主页</a></p>{{end}}
+  {{template "base/alert" .}}
+
+  {{if .HasOlder}}
+  <p><a href="/messages/with/{{.PeerID}}?before={{.OlderBefore}}">加载更早消息</a></p>
+  {{end}}
+
+  {{if .Messages}}
+  <ul class="j13-msg-thread">
+    {{range .Messages}}
+    <li class="j13-msg-bubble{{if .FromSelf}} is-self{{end}}{{if .IsSystem}} is-system{{end}}">
+      {{if .Subject}}<div class="j13-msg-bubble__subj">{{.Subject}}</div>{{end}}
+      <div class="j13-msg-bubble__body">{{.Content}}</div>
+      <div class="j13-muted">{{.CreatedLabel}}{{if .Kind}} · {{.Kind}}{{end}}</div>
+    </li>
+    {{end}}
+  </ul>
+  {{else}}
+  <p class="j13-empty">还没有消息。{{if .CanReply}}发送第一条私信开始对话。{{end}}</p>
+  {{end}}
+
+  {{if .CanReply}}
+  <form method="post" action="/messages/with/{{.PeerID}}" class="j13-form j13-msg-compose">
+    <input type="hidden" name="_csrf" value="{{.CSRF}}"/>
+    <label>回复
+      <textarea name="content" rows="4" required maxlength="4000">{{.Draft}}</textarea>
+    </label>
+    <button type="submit">发送</button>
+  </form>
+  {{end}}
+</main>
+{{template "base/footer" .}}
+{{end}}