feat: 新增5部经典下拉选经、TTS重试、PWA可安装

- 新增阿弥陀经/普门品/道德经/清静经/太上感应篇（维基文库底本，OpenCC转简体）
- 经典切换改为下拉框（佛/道分组），移动端复用底部面板并显示分组标题
- TTS 请求失败自动重试3次；普门品偈颂拆分修复600字上限问题
- 服务端缓存原子写（.tmp+rename），上限放宽至800字
- PWA：manifest + Service Worker，安装后无地址栏独立窗口
- 偏好记忆/逐字光标/回到诵读处/夜间模式等此前改动一并入库

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

manifest.json

@@ -0,0 +1,17 @@
+{
+  "name": "经典诵读",
+  "short_name": "经典诵读",
+  "description": "心经、金刚经、阿弥陀经、普门品、道德经、清静经、太上感应篇诵读——千问语音合成，晨钟暮鼓配乐，逐段跟读高亮。",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "portrait",
+  "background_color": "#f6f1e6",
+  "theme_color": "#f6f1e6",
+  "lang": "zh-CN",
+  "icons": [
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-180.png", "sizes": "180x180", "type": "image/png", "purpose": "any" }
+  ]
+}