refactor: manifest迁移到ota/文件夹

- ota/manifest.json (stable)
- ota/manifest-beta.json
- ota/manifest-nightly.json
- MainActivity URL更新为ota/路径

packages/client-android/android/app/src/main/java/dev/yuzu/gca/MainActivity.kt

@@ -101,7 +101,7 @@ class MainActivity : AppCompatActivity() {
     }
 
     private fun manifestUrl(channel: String): String {
-        val base = "https://git.childish-ghost.com/LukeMackin/Yuzu-GCA/raw/branch/main/packages/client-android"
+        val base = "https://git.childish-ghost.com/LukeMackin/Yuzu-GCA/raw/branch/main/ota"
         return when (channel) {
             "beta" -> "$base/manifest-beta.json"
             "nightly" -> "$base/manifest-nightly.json"