enhance(client): ユーザー名についてのヒントを追加

src/client/components/dialog.vue

@@ -10,6 +10,7 @@
 			<i v-else-if="type === 'warning'" class="fas fa-exclamation-triangle"></i>
 			<i v-else-if="type === 'info'" class="fas fa-info-circle"></i>
 			<i v-else-if="type === 'question'" class="fas fa-question-circle"></i>
+			<i v-else-if="type === 'help'" class="fas fa-question-circle"></i>
 			<i v-else-if="type === 'waiting'" class="fas fa-spinner fa-pulse"></i>
 		</div>
 		<header v-if="title"><Mfm :text="title"/></header>

src/client/components/signup.vue

@@ -6,7 +6,7 @@
 			<template #prefix><i class="fas fa-key"></i></template>
 		</MkInput>
 		<MkInput v-model="username" type="text" pattern="^[a-zA-Z0-9_]{1,20}$" :autocomplete="Math.random()" spellcheck="false" required @update:modelValue="onChangeUsername" data-cy-signup-username>
-			<template #label>{{ $ts.username }}</template>
+			<template #label>{{ $ts.username }} <div class="_button _help" v-tooltip:dialog="$ts.usernameInfo"><i class="far fa-question-circle"></i></div></template>
 			<template #prefix>@</template>
 			<template #suffix>@{{ host }}</template>
 			<template #caption>