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>

packages.txt

@@ -11,6 +11,7 @@ brightnessctl
 pamixer
 gtklock
 swayidle
+snixembed
 ttf-jetbrains-mono-nerd
 network-manager-applet
 polkit-gnome