feat: add user settings page (change username/password) and remove public registration

app/templates/register.html

@@ -1,20 +0,0 @@
-{% extends "base.html" %}
-{% block title %}注册 - Postcard Manager{% endblock %}
-{% block content %}
-<div class="auth-form">
-    <h2>注册</h2>
-    {% if error %}
-    <div class="alert alert-error">{{ error }}</div>
-    {% endif %}
-    <form method="post" action="/register">
-        <label>用户名</label>
-        <input type="text" name="username" required autofocus>
-        <label>密码</label>
-        <input type="password" name="password" required>
-        <label>确认密码</label>
-        <input type="password" name="password2" required>
-        <button type="submit" class="btn btn-primary">注册</button>
-    </form>
-    <p class="auth-link">已有账号？<a href="/login">登录</a></p>
-</div>
-{% endblock %}