fix: discover Gitea update source from release notes (no hardcode)

Gitea URL removed from source code. UpdateChecker now only hardcodes
GitHub API; the Gitea mirror URL is embedded in release notes as an
HTML comment (<!-- update-source: ... -->) and discovered at runtime.
Keeps self-hosted server address out of the public codebase.

version 1.8.0 -> 1.8.1 (code 17 -> 18)

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 = 17
-        versionName = "1.8.0"
+        versionCode = 18
+        versionName = "1.8.1"
     }
 
     signingConfigs {