Fix Decodex App login panel jump

[yvette-carlisle]

Summary

• Stabilize Decodex App login floating-panel placement across streaming login state updates.
• Keep existing panel position when the menu-bar parent window disappears instead of falling back to mouse-based placement.
• Add SwiftPM tests for panel sizing and placement regression coverage.

Verification

• swift test --package-path apps/decodex-app
• cargo make test-decodex-app-stage
• apps/decodex-app/script/build_and_run.sh --verify
• git diff --check