feat: 初始版本，圆角主题与首次管理员引导

app/templates/activities/submit.html

@@ -0,0 +1,10 @@
+{% extends 'base.html' %}
+{% block title %}投稿{% endblock %}
+{% block content %}
+<h3>向 {{ activity.title }} 投稿</h3>
+<form method="post" enctype="multipart/form-data">
+  <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
+  <div class="mb-3"><label class="form-label">图片</label><input class="form-control" name="images" type="file" accept="image/*" multiple required></div>
+  <button class="btn btn-primary" type="submit">提交</button>
+</form>
+{% endblock %}