improve PWA feel on iOS: offline fallback, overscroll, tap feedback, dark status bar, apple touch icon

Add offline.html served by the service worker when a never-visited route
is requested without network. Disable rubber-band overscroll bounce and
gray tap highlights so standalone mode feels less like a webview. Split
theme-color by prefers-color-scheme so the iOS status bar matches dark
mode, and add a proper 180x180 apple-touch-icon instead of downscaling
the 192px icon.

app/templates/base.html

@@ -6,10 +6,12 @@
   <title>{% block title %}습관 트래커{% endblock %}</title>
 
   <link rel="manifest" href="/static/manifest.json" />
-  <meta name="theme-color" content="#d97757" />
+  <meta name="theme-color" content="#d97757" media="(prefers-color-scheme: light)" />
+  <meta name="theme-color" content="#e08962" media="(prefers-color-scheme: dark)" />
   <meta name="apple-mobile-web-app-capable" content="yes" />
   <meta name="apple-mobile-web-app-status-bar-style" content="default" />
   <meta name="apple-mobile-web-app-title" content="습관 트래커" />
+  <link rel="apple-touch-icon" sizes="180x180" href="/static/icons/icon-apple-180.png" />
   <link rel="apple-touch-icon" href="/static/icons/icon-192.png" />
   <link rel="icon" href="/static/icons/icon-192.png" />
 
@@ -37,6 +39,7 @@
       </div>
       {% if current_user %}
       <div class="nav-user">
+        {% if account_level %}<span class="nav-level-pill">Lv.{{ account_level.level }}</span>{% endif %}
         <span>{{ current_user.name or current_user.email }}</span>
         <form method="post" action="/auth/logout">
           <button type="submit" class="btn-link">로그아웃</button>