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>

scripts/screenshot.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+FILE="$HOME/Pictures/screenshot-$(date +%s).png"
+mkdir -p "$HOME/Pictures"
+
+REGION=$(slurp) || exit 0
+grim -g "$REGION" - | satty \
+    --filename - \
+    --output-filename "$FILE" \
+    --copy-command wl-copy \
+    --early-exit