## Problem
Supplying any `platforms` table silently dropped all unlisted platforms,
making it easy to accidentally disable platforms. Invoking a disabled
platform produced no user-facing error.
## Solution
Switch to a merge model: all six platforms are enabled by default and
user entries are deep-merged on top. Set a platform key to `false` to
disable it explicitly. Add a `check_platform_enabled` guard in
`handle_command` for contest fetch, login, logout, and race actions.
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