Add Rivian

[kayleg]

Adds Rivian to plugins.txt.

Your Rivian in the bar: where it is, how full it is, and how far that gets you.

The bar shows the compass. Click it for a map of where the car is parked, which
way it is pointing, battery and range, lock and Gear Guard state, tyres,
odometer, and when the cloud last heard from the car.

Read-only, and it cannot flatten the battery: a Rivian pushes telemetry to
Rivian's cloud on its own schedule and this reads that cloud, so nothing here
ever touches the car.

• MIT, no build step — bash + QML, needs curl, jq, awk
• Signs in from the panel itself; no terminal step
• preview.png and screenshots/ are in the repo for the storefront
• 72 assertions run against saved gateway responses, so no account is needed to
  test it: ./tests/run

Derived from omarchy-tesla (MIT) —
the map, tile cache and geocoding come from there, with attribution in NOTICE.