Rename bg/fg palette keys to background/foreground

The canonical colors.toml keys are now background/foreground, including
all permutations (dark_background, darker_background, lighter_background,
dark_foreground, light_foreground, bright_foreground). The old short
names are gone entirely, not kept as aliases; legacy alacritty-generated
themes still resolve through the ANSI color0/color7 fallback.

Theme files are also regrouped: accent/selection/muted first, then the
backgrounds, then the foregrounds, then the named colors.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/omarchy-plymouth-set-by-theme

@@ -51,7 +51,7 @@ theme_color() {
   ' "$theme_dir/colors.toml"
 }
 
-bg=$(theme_color background bg)
-text=$(theme_color foreground fg)
+bg=$(theme_color background)
+text=$(theme_color foreground)
 
 exec omarchy-plymouth-set "$bg" "$text" "$theme_dir/unlock.png"