The Steam `LibraryProvider` keyed off `$HOME` + Linux paths, so the game
library was empty on Windows. Add Windows discovery: the default Steam
install dirs under Program Files (`ProgramFiles(x86)`/`ProgramFiles`/
`ProgramW6432`), with games on other drives picked up via each root's
`libraryfolders.vdf` — whose Windows values are backslash-escaped, so
unescape `\\` → `\`. The existing root-scan/dedup logic is shared via a
new `steam_roots_existing` helper. The custom store (mgmt JSON CRUD) was
already cross-platform; only Steam auto-discovery was Linux-only.
Not yet covered: a non-default Steam install dir (the registry
`Valve\Steam\InstallPath`). Degrades gracefully — no Steam → empty list.
clippy -D warnings + library tests green on Windows and Linux.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
c300deba9a