[Competition-View] feat(competition-view): terminal-style message log, font-size control and mock backend

[JavierRibaldelRio]

What changes

Improvements to the Competition View message area, plus a mock backend to exercise it without the pod.

Terminal-style messages

• The log is now in chronological order: new messages appear at the bottom and the view follows them.
• Scrolling up pauses the follow and shows a floating "Latest" button to jump back to the end.
• The jump to the bottom is instant: with smooth scrolling, a message arriving mid-animation was read as "scrolled away" and stalled the follow.
• The 500-message cap now drops the oldest entries instead of the newest.
• The Overview messages panel behaves the same way.

Readability

• New font-size slider (12–24 px), persisted in localStorage.
• The severity badge (INFO/OK/WARNING/FAULT) scales with the slider instead of staying fixed at 10 px, with a 10 px floor.
• In light theme, the WARNING and FAULT row backgrounds go from a 40 % tint (effectively white) to amber-100 / red-100, which are actually distinguishable. Dark theme is unchanged.

Mock backend

• frontend/competition-view/scripts/mock-backend.mjs: dependency-free server that stands in for the Go backend on port 4000. It sends messages across all four severities covering every payload variant, serves the orders catalog, publishes the vehicle state, and logs and validates every incoming order/send.
• Usage: pnpm mock:backend and, in another terminal, pnpm dev --filter competition-view.

Frontend-kit

• Adds the shadcn Slider component.