Pulls the three remaining platform-shaped interfaces out of app/common/
and into platform/network/, platform/leaderboard/ and platform/game/,
each with a function accessor + true no-op stub backend matching the
existing platform/profile/ template. Drops the duplicate
PlatformNetworkManagerInterface.h that was a stale copy of the same
type. Existing app-side singletons (LeaderboardManager,
LinuxLeaderboardManager, IPlatformNetworkStub, LinuxGame) keep working
unchanged - the structural fix is just that the interface they depend
on now lives in platform/ where it belongs.