Persist chat and image history

[devin-ai-integration]

Summary

Adds AsyncStorage persistence for both chat and image history so conversations and generated images survive app restarts.

Chat history (chat.tsx):

• Loads per-model chat states from rnai-chatStates on mount
• Persists to AsyncStorage only at stream completion ([DONE]) and on clear — avoids excessive writes during SSE streaming
• Guards chat() with chatLoaded flag to prevent race conditions during async load
• Clearing a chat removes that model's entry from storage

Image history (images.tsx):

• Loads image generation history from rnai-imageHistory on mount
• Saves after each successful image generation
• Guards generate() with imagesLoaded flag to prevent race conditions
• Clearing prompts removes the persisted data

Uses the same AsyncStorage pattern already established in App.tsx for theme/model preferences.

Review & Testing Checklist for Human

• Start a chat conversation, close the app, reopen — messages should be restored
• Switch between models, verify each model's history persists independently
• Generate an image, close the app, reopen — the image prompt and result should be restored
• Use "Clear chat" / "Clear prompts" and verify the history is actually removed (doesn't reappear on restart)

Notes

• Storage keys: rnai-chatStates (chat), rnai-imageHistory (images)

Link to Devin session: https://app.devin.ai/sessions/3ca51d55be7e4c4aa48cf5ab6ebcf95b
Requested by: @dabit3

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[devin-ai-integration]

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