Security audit finding SEC-IPC-03. The thumbnail IPC command resolved
`assets/{game_id}.jpg` from the resource directory without checking the
ID, unlike every other command that maps a game ID to a path. A crafted
ID containing path separators could therefore read any `.jpg` reachable
from the resource root. The handler also still carried a `dbg!` that
printed the resolved path to stderr in release builds.
The command now rejects anything that is not a single normal path
component with an `InvalidInput` I/O error, using the same
`is_single_component_game_id` gate as run_game and start_server. The
frontend already treats a failed thumbnail request as "no thumbnail".
Test plan: `just clippy`, `just test`. In the app, thumbnails for
catalog games still load; an invoke with game_id "../x" is rejected.
Claude-Session: https://claude.ai/code/session_017C3Nbgwpdm3YNwZhhFLHwg
ddidderr
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