feat(ui): add landing page with random public postcards for unauthenticated visitors

app/locales/en.json

@@ -3,6 +3,10 @@
     "name": "English"
   },
   "brand": "📮 Mailova",
+  "index.title": "Mailova",
+  "index.subtitle": "Postcards from around the world",
+  "index.go_login": "Login",
+  "index.empty": "No public postcards yet",
   "nav.profiles": "Profiles",
   "nav.settings": "Settings",
   "nav.invites": "Invites",

app/locales/zh.json

@@ -3,6 +3,10 @@
     "name": "中文"
   },
   "brand": "📮 星笺",
+  "index.title": "星笺",
+  "index.subtitle": "来自世界各地的明信片",
+  "index.go_login": "登录",
+  "index.empty": "暂无公开明信片",
   "nav.profiles": "Profile",
   "nav.settings": "设置",
   "nav.invites": "邀请码管理",