Replace track switch cooldown with race-activity check

[LinaBell]

Summary

• Replaced the 30-minute track switch cooldown (based on trackLastSwitchedDate) with a 5-minute check based on lastRaceStartedDate, which records when any user last started a race
• Non-admins are blocked from switching tracks if a race was started < 5 minutes ago (disabled button with tooltip)
• Admins see a confirmation modal warning that a race may be in progress but can proceed
• Removed trackLastSwitchedDate from the client state and switch track flow

Test plan

• Start a race and confirm lastRaceStartedDate is set on the world data object
• As a non-admin, navigate to Switch Track within 5 minutes of a race starting — button should be disabled with a hover tooltip
• As an admin, navigate to Switch Track within 5 minutes — button is enabled, clicking shows a confirmation modal warning about active races
• Wait 5+ minutes after last race start — any user can switch tracks freely with no restrictions