fix: startup always checks + dual-source (Gitea+GitHub) take highest

- start(): check on every launch (no 24h throttle) so the banner shows
  after restart even if checked manually moments ago; plus a 24h loop
  while app is resident
- check both Gitea and GitHub release APIs, skip whichever is
  unreachable, notify the higher version
- version 1.7.2 -> 1.7.3 (code 15 -> 16)

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 = 15
-        versionName = "1.7.2"
+        versionCode = 16
+        versionName = "1.7.3"
     }
 
     signingConfigs {