Fix Android backup with missing journal

[limeimetw]

Summary

• Fix Android backup creation when the optional SQLite rollback journal (lime.db-journal) is absent or transient.
• Propagate backup/write failures from DBServer.backupDatabase() so the UI shows failure instead of success/0 B output.
• Ensure backup progress is hidden on both success and failure.
• Add regression coverage for missing/present optional journal handling and output failure propagation.

Verification

• ANDROID_HOME=$HOME/Android/Sdk ANDROID_SDK_ROOT=$HOME/Android/Sdk ./gradlew :app:compileDebugJavaWithJavac :app:compileDebugAndroidTestJavaWithJavac
• Attempted targeted connected androidTest for the new DBServer tests; blocked by no connected Android device (DeviceException: No connected devices!).
• Independent review performed; initial blockers fixed before PR.

Fixes #94