fix: scramble update-source (XOR+reverse+Base64) + hide in UI

- update-source in release notes now XOR-scrambled (not plain Base64)
- UpdateDialog filters out update-source comment line from notes
- Download progress shows "正在下载更新…" instead of percentage/source
- version 1.8.1 -> 1.8.2 (code 18 -> 19)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -13,8 +13,8 @@ android {
         applicationId = "com.a2i.forwarder"
         minSdk = 34
         targetSdk = 36
-        versionCode = 18
-        versionName = "1.8.1"
+        versionCode = 19
+        versionName = "1.8.2"
     }
 
     signingConfigs {