Improve bottom banner ad layering and loading state

[MihaiCristianCondrea]

Summary

• ensure the bottom banner ad is only shown for the bottom navigation layout and always sits above the bar
• add a reusable helper to manage banner visibility and prevent duplicate ad load calls while improving the loading indicator handling
• refresh the bottom banner layout to use the shared Material card styling and update the main screen layout for the new progress indicator

Testing

• ./gradlew test (fails: SDK location not configured in CI environment)

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https://chatgpt.com/codex/tasks/task_e_68ce5f2514a8832dbbdd32785459d96e