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>

niri/config.kdl

@@ -83,7 +83,7 @@ binds {
     Mod+Shift+8 { move-window-to-workspace 8; }
     Mod+Shift+9 { move-window-to-workspace 9; }
 
-    Print { spawn-sh "grim -g \"$(slurp)\" ~/Pictures/screenshot-$(date +%s).png"; }
+    Print { spawn "screenshot"; }
 
     Mod+M { move-window-to-workspace "minimized"; }
     Mod+Shift+M { focus-workspace "minimized"; }