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/development.md

@@ -64,6 +64,11 @@ modules/
                                of Flatpaks/host files in <runtimeDir>/steam-frame-nix/applications
     check.nix, fixtures/       test: real entries rewritten (fixtures/expected), generator runs
   docker.nix                   service: rootless dockerd (docker)
+  screenshots.nix              ~/Pictures link to the SteamVR screenshots (screenshots)
+  screenshots/
+    link.sh, package.nix       service steam-frame-nix-screenshots (+ .path), switch:
+                               <runtimeDir>/steam-frame-nix/screenshots -> account folder
+    check.nix                  test: account detection on fake Steam dirs
   firefox.nix                  Flatpak prefs extension and launcher (firefox)
   firefox/wrapper.nix          app: launcher wrapper (desktop profile, fullscreen fix)
   firefox/check.nix            test: wrapper against a fake flatpak
@@ -128,6 +133,7 @@ they are kept even where a file name says more:
 | `frame-controls` | `dashboard.frameControls` | `frame-controls` (SteamVR, state) | `steam-ui-patches` |
 | `steamvr-debugger` | `steamvrDebugger` | | `steamvr-webhelper-debugger` |
 | `launchers` | `launchers` | | `steam-frame-nix-launchers` (`.service`, `.path`) |
+| `screenshots` | `screenshots` | | `steam-frame-nix-screenshots` (`.service`, `.path`) |
 
 Log of all patches:
 `journalctl --user -u steam-ui-patches -u steam-keyboard-patch -u vr-keyboard-relay`.