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>

themes/vantablack/colors.toml

@@ -1,15 +1,16 @@
 mode = "dark"
 
 accent = "#8d8d8d"
-
-bg = "#000000"
-dark_bg = "#090909"
-darker_bg = "#070707"
-lighter_bg = "#000000"
 selection = "#1a1a1a"
-fg = "#ffffff"
-light_fg = "#ececec"
-bright_fg = "#ffffff"
+
+background = "#000000"
+dark_background = "#090909"
+darker_background = "#070707"
+lighter_background = "#000000"
+
+foreground = "#ffffff"
+light_foreground = "#ececec"
+bright_foreground = "#ffffff"
 
 red = "#a4a4a4"
 yellow = "#cecece"