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-theme-colors-from-alacritty

@@ -129,8 +129,8 @@ cat > "$COLORS_OUTPUT" <<EOF
 accent = "$accent"
 selection = "$selection_background"
 
-bg = "$background"
-fg = "$foreground"
+background = "$background"
+foreground = "$foreground"
 
 color0 = "$color0"
 color1 = "$color1"