screenshot: revert flameshot, use grim+slurp+satty pipeline

Flameshot's portal-based capture path on niri triggers an
xdg-desktop-portal access prompt on every invocation, blocking
the screenshot itself. grim talks to wlr-screencopy directly and
never touches the portal, so no prompt.

Pipeline: slurp (region) → grim (capture) → satty (annotate +
copy + save). satty replaces the wofi action menu by being the
post-capture surface itself.

- scripts/screenshot.sh added
- install.sh symlinks it; flameshot.ini generator removed
- niri Mod+Print → "screenshot"
- packages.txt drops flameshot
- docs tables updated

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -13,7 +13,7 @@ A personal Wayland desktop configuration centered around the **Niri** tiling com
 | Lockscreen | gtklock |
 | Login/greeter | greetd + regreet |
 | Wallpaper | swww |
-| Screenshots | flameshot |
+| Screenshots | grim + slurp + satty |
 | Clipboard | cliphist |
 | Theme | Tomorrow Night (GTK: Materia-dark, icons: Papirus) |