Miscellaneous improvements to the travel_app behavior and appearance

[jacobsimionato]

I couldn't resist a few drive-by fixes, mostly to the prompt context:

• Make icons work by being more clear to the LLM to use travel_icons
• Make image compulsory in itinerary_item. Otherwise they look weird.
• Make card elevation 0
• Try to discourage hallucinating image URLs

[gemini-code-assist]

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