feat: 简航 SimpleNav 个人网站导航系统（PHP + SQLite）

- 前台：搜索引擎切换+联想下拉、网格板块（常用/工具）、合集标签页
- 后台：初始化/登录、网站/板块/引擎增删改、上下移排序
- 添加网站自动抓取名称与图标（存储于服务器），支持自定义简称
- 一键全部更新（批量刷新图标+名称，保留自定义简称）
- 搜索联想服务端代理（必应/百度/谷歌/搜狗/360/DuckDuckGo）
- 零依赖：原生 CSS/JS，PDO SQLite

site.webmanifest

@@ -0,0 +1,15 @@
+{
+  "name": "简航 SimpleNav",
+  "short_name": "简航",
+  "start_url": "./",
+  "scope": "./",
+  "display": "standalone",
+  "background_color": "#d8d9da",
+  "theme_color": "#2F6BF7",
+  "icons": [
+    { "src": "assets/icons/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "assets/icons/icon-512.png", "sizes": "512x512", "type": "image/png" },
+    { "src": "assets/icons/icon-192-maskable.png", "sizes": "192x192", "type": "image/png", "purpose": "maskable" },
+    { "src": "assets/icons/icon-512-maskable.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
+  ]
+}