feat: support Chinese nickname in registration

- User input "昵称" stored in `name` field (supports Chinese)
- `username` auto-generated (PocketBase requires ASCII)
- Password rules displayed inline with real-time validation
- All UI displays prefer `name` over `username`

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/views/Home.vue

@@ -70,7 +70,7 @@ function openGameDetail(game: Game) {
     <section class="welcome-bar">
       <div class="welcome-left">
         <h1 class="welcome-title">
-          欢迎回来, <span class="gg-text-gradient">{{ userStore.user?.username || '玩家' }}</span>
+          欢迎回来, <span class="gg-text-gradient">{{ userStore.user?.name || userStore.user?.username || '玩家' }}</span>
         </h1>
         <div class="status-line">
           <span :class="statusDotClass" />