Add Rivian logo and update related functions

[bballdavis]

Added Rivian Logo, both as an image asset (Base64) as well as the related support functions for selection.

[tidbyt]

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[bballdavis]

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[Copilot]

Pull request overview

This PR adds Rivian brand support to the EV logo selection feature by introducing the Rivian logo as a base64-encoded image asset and updating the related support functions.

Changes:

• Added RIVIAN_LOGO_18x18 base64-encoded image constant
• Updated get_ev_logo() function to return Rivian logo when "RIVIAN" is selected
• Added "Rivian" option to the EV Brand dropdown in the schema, positioned alphabetically between Renault and Seat

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[Copilot]

The README.md documentation lists available EV brands (lines 224-234) but does not include Rivian. Since this PR adds Rivian as a new brand option, the documentation should be updated to include it in the list. Rivian should be added between "Renault" and "Seat" to maintain alphabetical order.

[bballdavis]

I've updated the readme.