fix: add ~/.steam/debian-installation/ to Steam path detection

[fernandomema]

Problem

DetectSteamPath() in src/platform/linux/cloud_intercept.cpp currently searches these directories (in order):

1. ~/.local/share/Steam/
2. ~/.var/app/com.valvesoftware.Steam/.local/share/Steam/
3. ~/.steam/steam/

On Debian/Ubuntu systems (and derivatives like Linux Mint, Pop!_OS, etc.), the Steam package installs to ~/.steam/debian-installation/ by default. This path is not in the detection list.

When the path is not found, LoadAccountIdFromLoginUsers() fails to open loginusers.vdf, leaving accountId at 0. Every cloud hook then checks:

uint32_t accountId = CloudIntercept::GetAccountId();
if (accountId == 0) {
    return origFn(...);  // falls through to Steam's own cloud
}

This effectively disables CloudRedirect for all namespace apps — cloud RPCs pass straight through to Steam servers, which return Upload Access Denied for apps that the user doesn't own.

Fix

Add ~/.steam/debian-installation/ as a fallback between the Flatpak path and ~/.steam/steam/. The path is only used when the directory exists, so no change in behavior for other distros.

The detection order becomes:

1. ~/.local/share/Steam/ (XDG default)
2. ~/.var/app/com.valvesoftware.Steam/.local/share/Steam/ (Flatpak)
3. ~/.steam/debian-installation/ (Debian/Ubuntu package)
4. ~/.steam/steam/ (manual/other)

Testing

Confirmed on a Debian-based system where ~/.steam/debian-installation/ is the active Steam directory. After applying this fix, accountId bootstraps correctly from loginusers.vdf and CloudRedirect intercepts cloud RPCs as expected.

[Selectively11]

Oops. Thanks, will review/merge/release.