feat: add email-based authentication

templates/_register_form.html

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+<form method="post" class="space-y-4"><input type="hidden" name="csrf_token" value="{{ csrf_token() }}"><label class="block text-sm font-semibold">显示名<input name="username" required {{ 'disabled' if not allow_registration }} class="mt-1 w-full rounded-lg border border-line px-3 py-2"></label><label class="block text-sm font-semibold">邮箱<input name="email" type="email" required {{ 'disabled' if not allow_registration }} class="mt-1 w-full rounded-lg border border-line px-3 py-2" autocomplete="email"></label><label class="block text-sm font-semibold">密码（12 至 128 位）<input name="password" type="password" minlength="12" required {{ 'disabled' if not allow_registration }} class="mt-1 w-full rounded-lg border border-line px-3 py-2" autocomplete="new-password"></label><label class="block text-sm font-semibold">图形验证码<div class="mt-1 flex gap-2"><input name="captcha" required {{ 'disabled' if not allow_registration }} class="min-w-0 flex-1 rounded-lg border border-line px-3 py-2"><span class="rounded-lg bg-blueSoft px-3 py-2 font-bold tracking-widest">{{ captcha }}</span><a class="rounded-lg border border-line px-2 py-2" href="{{ url_for('main.register') }}">↻</a></div></label>{% if not allow_registration %}<p class="text-sm text-dangerText">当前未开放自助注册，请联系管理员。</p>{% endif %}<button {{ 'disabled' if not allow_registration }} class="ui-btn ui-btn-lg ui-btn-primary w-full">发送邮箱验证码</button></form>