SteamVR screenshots linked into ~/Pictures

Steam keeps VR screenshots (Steam button + trigger) in
~/.local/share/Steam/userdata/<account ID>/760/remote/250820/screenshots,
where Dolphin, Gwenview and the file pickers don't look.

screenshots (module screenshots, opt-in) links ~/Pictures/<name> (default
"SteamVR Screenshots") there. The account ID comes from steamUserId, or,
unset, at runtime: the Home Manager link points to
<runtimeDir>/steam-frame-nix/screenshots (tmpfs), which the oneshot user
service steam-frame-nix-screenshots points to the account last logged in
(loginusers.vdf MostRecent, else latest timestamp, else the only userdata
folder), on switch, at login and when loginusers.vdf or userdata changes.

install.sh cleanup knows the tmpfs link (--keep screenshots while used).
Flake checks: account detection on fake Steam dirs; cleanup of the link.

docs/cleanup.md

@@ -26,6 +26,7 @@ is reported as "left alone" and never touched:
 | `jellyfin` | the hwdec shim entries in the Jellyfin Flatpak's user override `~/.local/share/flatpak/overrides/org.jellyfin.JellyfinDesktop` (nix-flatpak), an empty override file, and the shim copy of earlier versions in `~/.var/app/org.jellyfin.JellyfinDesktop` (marker `~/.local/state/steam-frame-nix/jellyfin-hwdec-shim`) |
 | `ui-state`: `~/.local/state/steam-frame-nix/ui-patches/<name>.json` | the dashboard patches' saved choices; `--all` only, never `--orphans`; stray `*.json.tmp` files |
 | `launchers`: `/run/user/1000/steam-frame-nix/applications` | the entries of [launchers](launchers.md) (tmpfs); `--all` only (switches remove those of removed launchers themselves) |
+| `screenshots`: `/run/user/1000/steam-frame-nix/screenshots` | a link to `*/userdata/*/760/remote/250820/screenshots` ([SteamVR screenshots](screenshots.md)); kept by `--orphans --keep screenshots` (while `steamUserId` is unset) |
 | `dirs` | `~/.local/state/steam-frame-nix` and `/run/user/1000/steam-frame-nix` when empty |
 
 ### Left by older versions