Add safety guards, setup guide, and material dependency

[OgeonX-Ai]

Summary

• add guardrails to skip TTS requests when prompt or voice is blank
• prevent playback attempts when audio files are missing to avoid errors
• centralize backend calls behind a lifecycle-aware helper to manage processing state and resolve the merge conflict in MainActivity
• add a LOCAL_SETUP guide detailing backend setup, Android configuration, and customization steps
• add the Material Components dependency so Theme.Material3 XML resources resolve correctly

Testing

• Not run (not requested)

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Codex Task