feat(invite): add invite code registration system

app/templates/register.html

@@ -0,0 +1,23 @@
+{% extends "base.html" %}
+{% block title %}注册 - 星笺{% endblock %}
+{% block content %}
+<div class="auth-form">
+    <h2>注册</h2>
+    <p style="color:var(--text-muted);font-size:.85rem;margin-bottom:1rem">邀请码由已有用户或管理员提供</p>
+    {% if error %}
+    <div class="alert alert-error">{{ error }}</div>
+    {% endif %}
+    <form method="post" action="/register">
+        <label>邀请码</label>
+        <input type="text" name="invite_code" required maxlength="8" placeholder="8位字母数字" style="text-transform:none" autofocus>
+        <label>用户名</label>
+        <input type="text" name="username" required maxlength="64">
+        <label>密码</label>
+        <input type="password" name="password" required minlength="6">
+        <label>确认密码</label>
+        <input type="password" name="confirm_password" required>
+        <button type="submit" class="btn btn-primary">注册</button>
+    </form>
+    <p style="margin-top:1rem;text-align:center;font-size:.85rem">已有账号？<a href="/login">去登录</a></p>
+</div>
+{% endblock %}