fix: download timeout + file validation + source indicator + stale APK

Root cause of "install doesn't update": gradle cached old APK, release
uploaded stale version. Fixes:
- Dedicated download OkHttpClient with 120s readTimeout (was 30s)
- 64KB buffer (was 8KB) for faster large-file download
- Post-download file size validation (Content-Length / release declared)
- Reject files < 1MB (likely HTML error pages)
- downloadSource StateFlow shows "Gitea" or "GitHub" during download
- UpdateDialog shows "正在下载更新…（来自 Gitea）"
- version 1.9.2 -> 1.9.3 (code 31 -> 32)

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 = 30
-        versionName = "1.9.1"
+        versionCode = 32
+        versionName = "1.9.3"
     }
 
     signingConfigs {