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.

dotfiles/zz-blankpad/niri/.config/niri/cfg/misc.kdl

@@ -1,5 +1,9 @@
     prefer-no-csd
-    screenshot-path null
+    // Default save path for niri-native captures (screenshot UI,
+    // screenshot-screen). scripts/screenshot-desk.sh (Mod+Shift+S) writes
+    // one file per monitor here; flameshot full is dead on 2+ outputs
+    // docked — see omp-docs/tools/niri/screenshots.md.
+    screenshot-path "~/Pictures/Screenshots/Screenshot from %Y-%m-%d %H-%M-%S.png"
 
     environment {
         ELECTRON_OZONE_PLATFORM_HINT "auto"