Use VSnackbarQueue for stacking notifications

[PaulHax]

Summary

• Replace VSnackbar with VSnackbarQueue to support multiple stacking notifications
• Move snackbar position from bottom-left to bottom-right
• Simplify AlertsController to push messages to a queue array

Test plan

• Start the app and trigger multiple decisions in quick succession
• Verify notifications stack and display sequentially
• Verify notifications appear in bottom-right corner

[PaulHax]

closed in favor of #80