feat(steam): share game library with gaming users via bind mounts

[hikaps]

Summary

• Add Steam library folder sharing so gaming users don't need to re-download games already installed by the compositor user
• Parse libraryfolders.vdf (text VDF format), mount steamapps/common via bind mounts, copy app manifests, and generate a target libraryfolders.vdf with full apps section so Steam recognizes installed games
• Add settings toggle in the Steam section of Settings page
• Gate on target user having launched Steam at least once (getTargetSteamUserId check)
• Clean up shared state on session stop (restore minimal libraryfolders.vdf after bind mounts are torn down)

Changes

• SteamConfigManager: SteamLibraryFolder struct, parseLibraryFoldersVdf, generateLibraryFoldersVdf (with apps/label/totalsize), shareLibraryToUser, cleanupLibrarySharing, loadLibraryFolders
• SessionRunner: load library folders once before instance loop, call shareLibraryToUser per-instance, call cleanupLibrarySharing in stop()
• SettingsPage: "Share game library with players" checkbox with tooltip

Technical details

• Library 0 maps to target's Steam root; additional libraries go to ~/.couchplay/steam-libs/<i>/
• Mount specs use "source|alias" format via CouchPlayHelperClient::mountSharedDirectories
• ACLs set on source library paths via setPathAclWithParents
• Generated VDF includes apps {} block with all source app IDs so Steam shows games as installed