feat: 渠道选择实装 — v6读取manifest-{channel}.json

- checkUpdate根据选中渠道读取manifest-stable/beta/nightly.json
- 三个manifest使用统一H5字段: h5_version/h5_url/h5_sha256
- 版本号比对可独立推进各渠道

ota/manifest-nightly.json

@@ -1 +1 @@
-{"version":"0.1.0","builds":{"android":{"type":"apk","tag":"v0.1.0-dav-android-8","url":"https://git.childish-ghost.com/LukeMackin/Yuzu-GCA/releases/download/v0.1.0-dav-android-8/v0.1.0-dav-android-8-debug.apk","sha256":"b2451a398ea2dd34e585c5ec4529d6654451c9b9ab4ddf4a25ec9b032f0ac2bf","size":3506569,"channel":"nightly","minVersion":"0.1.0"}}}
+{"h5_version":"0.1.0-dav-android-6","h5_sha256":"6a25c8c5c1e2bacdac416885a0cd4754debf699344d63673cffcc91b5d646295","h5_url":"https://git.childish-ghost.com/LukeMackin/Yuzu-GCA/raw/branch/main/ota/h5/index-v6.html"}