Housekeeping: fix theme SSOT, wire screenshot, remove dead swaylock

- mako/config: generate from theme/colors.json in install.sh instead of
  symlinking a static file — mako is the only tool that can't use CSS
  variables, so colors.json is now the single source of truth for it
- gtklock/config.ini: remove hardcoded /home/alex absolute path for style;
  gtklock falls back to $XDG_CONFIG_HOME/gtklock/style.css by default
- niri: replace inline grim spawn-sh with `spawn "screenshot"` to use the
  existing script (which also copies to clipboard via wl-copy)
- install.sh: add screenshot symlink; mako generation step
- packages.txt: add snixembed (used in niri spawn-at-startup, was missing)
- swaylock/config: delete (replaced by gtklock in previous commit)
- .gitignore: add mako/config (now generated, not tracked)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

install.sh

@@ -30,7 +30,17 @@ mkdir -p ~/.config/theme
 ln -sf "$(pwd)/theme/colors.css" ~/.config/theme/colors.css
 ln -sf "$(pwd)/wofi/config" ~/.config/wofi/config
 ln -sf "$(pwd)/wofi/style.css" ~/.config/wofi/style.css
-ln -sf "$(pwd)/mako/config" ~/.config/mako/config
+python3 - <<'PYEOF'
+import json, os
+c = json.load(open("theme/colors.json"))
+config = f"""background-color={c['background']}
+text-color={c['foreground']}
+border-size=2
+border-color={c['blue']}
+default-timeout=4000
+"""
+open(os.path.expanduser("~/.config/mako/config"), "w").write(config)
+PYEOF
 ln -sf "$(pwd)/gtklock/config.ini" ~/.config/gtklock/config
 ln -sf "$(pwd)/gtklock/style.css" ~/.config/gtklock/style.css
 ln -sf "$(pwd)/fish/config.fish" ~/.config/fish/config.fish
@@ -48,6 +58,7 @@ ln -sf "$(pwd)/scripts/powermenu.sh" ~/.local/bin/powermenu
 ln -sf "$(pwd)/scripts/clipboard.sh" ~/.local/bin/clipboard-picker
 ln -sf "$(pwd)/scripts/power-profile.sh" ~/.local/bin/power-profile
 ln -sf "$(pwd)/scripts/fan-profile.sh" ~/.local/bin/fan-profile
+ln -sf "$(pwd)/scripts/screenshot.sh" ~/.local/bin/screenshot
 
 echo "==> Enabling systemd user services"
 mkdir -p ~/.config/systemd/user