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/catppuccin/colors.toml

@@ -1,17 +1,18 @@
 mode = "dark"
 
 accent = "#89b4fa"
-
-bg = "#1e1e2e"
-dark_bg = "#161622"
-darker_bg = "#101019"
-lighter_bg = "#313244"
 selection = "#45475a"
 muted = "#585b70"
-dark_fg = "#6c7086"
-fg = "#cdd6f4"
-light_fg = "#bac2de"
-bright_fg = "#cdd6f4"
+
+background = "#1e1e2e"
+dark_background = "#161622"
+darker_background = "#101019"
+lighter_background = "#313244"
+
+foreground = "#cdd6f4"
+dark_foreground = "#6c7086"
+light_foreground = "#bac2de"
+bright_foreground = "#cdd6f4"
 
 red = "#f38ba8"
 yellow = "#f9e2af"