fix: theme-aware favicon

[alerodriargui]

General Changes

• Fixes low-contrast Aave favicon in browser dark mode
• Adds a theme-aware SVG favicon using prefers-color-scheme
• Keeps the existing .ico favicon as a fallback for browsers that do not support SVG favicons

Developer Notes

Fixes: #2545
The new favicon.svg switches between a dark fill in light mode and a light fill in dark mode, improving tab visibility without changing the existing fallback favicon behavior.

Validation performed:

• Ran SVGO on the new SVG asset
• Confirmed the SVG parses as valid XML
• Verified Chrome headless renders the SVG in both light and forced dark mode
• Ran next lint; it completed with exit code 0, with existing CRLF formatting warnings from the Windows checkout

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Reviewer Checklist

Please ensure you, as the reviewer(s), have gone through this checklist to ensure that the code changes are ready to ship safely and to help mitigate any downstream issues that may occur.

• End-to-end tests are passing without any errors
• Code changes do not significantly increase the application bundle size
• If there are new 3rd-party packages, they do not introduce potential security threats
• If there are new environment variables being added, they have been added to the .env.example file as well as the pertinant .github/actions/* files
• There are no CI changes, or they have been approved by the DevOps and Engineering team(s)

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