🎨 Palette: Improve image accessibility

[cmonteverde]

💡 What: Added alt attributes to images embedded via raw HTML in app.py and satellite_homepage.py.
🎯 Why: Images embedded with st.markdown were missing alt text, making them inaccessible to screen readers.
♿ Accessibility:

• Added alt="" to the decorative topography background image.
• Added alt="CeCe Avatar" to the avatar image.
• Added alt="Climate Copilot Logo" to the logo image.
• Created .gitignore to prevent build artifacts from being committed.
• Updated UX journal in .Jules/palette.md.

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PR created automatically by Jules for task 7290889087096199472 started by @cmonteverde

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