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.

README.md

@@ -55,6 +55,7 @@ configuration, limitations and how it works.
 - [Jellyfin](docs/jellyfin.md) (`jellyfin.hardwareDecoding`): hardware video decoding in the Jellyfin Desktop Flatpak.
 - [Launchers](docs/launchers.md) (`launchers.<desktop ID>`): an app's own desktop entry (Flatpak, Nix package) with extra options, environment, wrappers and MIME defaults; `keyring` keeps its KDE wallet logins in both sessions (Electron too).
 - [Docker](docs/docker.md) (`docker`): rootless Docker as a user service, CLI working in both sessions.
+- [SteamVR screenshots](docs/screenshots.md) (`screenshots`): `~/Pictures/SteamVR Screenshots`, a link to Steam's folder of the current account.
 
 **Your own patches** of Steam's UI, and fixing patches after a Steam update: [UI patches](docs/ui-patches.md).
 
@@ -215,7 +216,7 @@ home-manager switch --flake .#steamos  # manual setup, from the flake's director
 In your own flake, add the input as above and
 `steam-frame-nix.homeManagerModules.default` to the modules. `default`
 imports all modules; single ones:
-`homeManagerModules.{session,portal,applications-menu,keyboard-layout,vr-keyboard-extra-keys,vr-keyboard,hidden-apps,steam-ui-patches,launcher-menu,steamvr-debugger,cleanup,dashboard-windows,steam-close-button,window-curvature,frame-controls,clipboard-sync,firefox,jellyfin,launchers,docker}`
+`homeManagerModules.{session,portal,applications-menu,keyboard-layout,vr-keyboard-extra-keys,vr-keyboard,hidden-apps,steam-ui-patches,launcher-menu,steamvr-debugger,cleanup,dashboard-windows,steam-close-button,window-curvature,frame-controls,clipboard-sync,firefox,jellyfin,launchers,docker,screenshots}`
 (`steam-keyboard-patch` and `keyring` still work as the former names of
 `vr-keyboard-extra-keys` and `launchers`). Every module imports `cleanup` (see
 [Changes outside Nix](#changes-outside-nix-exceptions)); which file is
@@ -326,6 +327,9 @@ which: [Repository layout](docs/development.md).
 | `steamFrame.docker.enable` | bool | `false` | Rootless Docker as a user service, CLI for both sessions, see [Docker](docs/docker.md). |
 | `steamFrame.docker.package` | package | `pkgs.docker` | Docker package (daemon and CLI). |
 | `steamFrame.docker.host` | str, read-only | `"unix://${runtimeDir}/docker.sock"` | The daemon's `DOCKER_HOST` (the CLI's default). |
+| `steamFrame.screenshots.enable` | bool | `false` | Link `~/Pictures/<name>` to the SteamVR screenshots, see [SteamVR screenshots](docs/screenshots.md). |
+| `steamFrame.screenshots.name` | str | `"SteamVR Screenshots"` | Name of the link in `~/Pictures`. |
+| `steamFrame.screenshots.steamUserId` | null or str (digits) | `null` | Steam account ID (folder in `~/.local/share/Steam/userdata`); `null`: the account last logged in, found at runtime. |
 | `steamFrame.cleanup.package` | package, read-only | | `steam-frame-nix-cleanup` (on `PATH` too), see [Changes outside Nix](#changes-outside-nix-exceptions). |
 
 Renamed options still work under their old names, with a warning:
@@ -363,6 +367,7 @@ lives in memory (the UI patches). These are written at runtime:
 | `/run/user/1000/systemd/user/steamvr.service.d/50-steam-frame-nix-debugger.conf`, `/run/user/1000/steam-frame-nix/steamvr-debugger-restore` | SteamVR debugger: puts the key back when SteamVR stops, without Nix | until reboot (tmpfs) | reboot; `steam-frame-nix-cleanup` while SteamVR is stopped and the debugger is off |
 | `~/.local/state/steam-frame-nix/ui-patches/<name>.json` | Saved choices of dashboard patches ([persistent state](docs/ui-patches.md#persistent-state)): window control bar placements (`frame-controls`), "Steam hidden" (`steam-close-button`). SteamOS's `steamvr.service` deletes `~/.cache/SteamVR` (the dashboard's own browser storage) on every SteamVR start. | until removed: kept when a patch is disabled (the choices come back when you enable it again) | `steam-frame-nix-cleanup --all`, `install.sh uninstall` |
 | `/run/user/1000/steam-frame-nix/applications/<id>.desktop` (with `.<id>.desktop.sum`, `.lock`) | [Launchers](docs/launchers.md) of Flatpaks and host files; `~/.local/share/applications/<id>.desktop` is a Home Manager link to it | until reboot (tmpfs), written again at login | reboot; the next switch or rewrite when the launcher or its app is gone; `steam-frame-nix-cleanup --all` |
+| `/run/user/1000/steam-frame-nix/screenshots` | [SteamVR screenshots](docs/screenshots.md#how-it-works) without `steamUserId`: link to the current account's folder; `~/Pictures/<name>` is a Home Manager link to it | until reboot (tmpfs), written again at login | reboot; `steam-frame-nix-cleanup` once unused |
 | mtime of `~/.local/share/applications` | [Launchers](docs/launchers.md#how-it-works): a running Steam rescans the "+" menu | only the directory's timestamp | nothing to remove |
 | mtime of `~/.local/share/icons/hicolor` | [icon fallbacks](docs/launcher-menu.md#icon-fallbacks): a running Steam rescans icons | only the directory's timestamp | nothing to remove |