Verify function return type consistency across matchmaking, matchhistory, and leaderboard modules

[Copilot]

Three compile errors caused by mismatches between .c implementations and .h declarations: int return types conflicting with bool declarations for isQueueEmpty, isUserInQueue, and isStackEmpty, plus a freeLeaderboard code block placed outside any function body in leaderboard.c.

Changes

• matchmaking.c: isQueueEmpty and isUserInQueue return bool, consistent with matchmaking.h
• matchhistory.c: isStackEmpty returns bool, consistent with matchhistory.h
• leaderboard.c: freeLeaderboard body is properly scoped within the function definition

// matchmaking.h
bool isQueueEmpty(MatchQueue* queue);
bool isUserInQueue(MatchQueue* queue, int user_id);

// matchmaking.c — now consistent
bool isQueueEmpty(MatchQueue* queue) { ... }
bool isUserInQueue(MatchQueue* queue, int user_id) { ... }

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