niri: fix screenshots on 2+ outputs — Mod+Shift+S captures all monitors

flameshot full dies docked: its portal path hits niri's
screenshot_all_outputs ensure!(outputs.len()==1) (upstream niri#4313,
niri 26.04), so every Print press on the home desk produces error
bubbles. niri's built-in screenshot UI selects only one output, so add
scripts/screenshot-desk.sh: per-monitor focus + screenshot-screen with
explicit --path, focus restored, one PNG per monitor in
~/Pictures/Screenshots. Bind Mod+Shift+S (Frank's call); noctalia
settings moves to Mod+Shift+O. Restore screenshot-path (was null, now
root-level as config.screenshot_path requires) for UI saves. Live-
verified docked: valid 1920x1080 PNGs for eDP-1 + DP-2, focus restored;
bats 27/27 incl. niri validate on both desk profiles.

omp-docs/tools/niri/keybindings.md

@@ -21,7 +21,7 @@
 | `Mod+B` | Open Browser: firefox |
 | `Mod+Shift+Return` | Open Wallpaper Selector: noctalia wallpaper toggle |
 | `Mod+S` | Open Control Center: noctalia controlCenter |
-| `Mod+Shift+S` | Open Settings: noctalia settings |
+| `Mod+Shift+O` | Open Settings: noctalia settings |
 | `Mod+D` | Open App Launcher: noctalia launcher |
 | `Mod+ALT+L` | Lock Screen: noctalia lock |
 | `Mod+Shift+Q` | Session Menu: noctalia sessionMenu |
@@ -166,6 +166,7 @@
 | `Print` | Screenshot: flameshot full |
 | `Ctrl+Print` | Screenshot: flameshot region |
 | `Alt+Print` | Screenshot: flameshot screen |
+| `Mod+Shift+S` | Screenshot: all monitors (per-monitor files) |
 
 ## Emergency Escape Key