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/index.md

@@ -91,6 +91,7 @@ Column-based scrollable tiling. Columns are cycled with Mod+Left/Right (or Mod+H
 | [[tips|tips.md]] | howto | Practical tips for working with niri |
 | [[displays|displays.md]] | reference | Desk-profile system: EDID-keyed outputs, hotplug auto-switch, portrait transforms, GPU notes |
 | [[keybindings|keybindings.md]] | reference | GENERATED keybinding cheat sheet (scripts/gen-keybinds.sh) |
+| [[screenshots|screenshots.md]] | reference | Screenshot bind matrix: flameshot vs niri-native, 2+ output limits, all-monitor capture |
 
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