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/last-horizon/colors.toml

@@ -1,22 +1,24 @@
 mode = "dark"
 
 accent = "#b59790"
+selection = "#584e51"
+muted = "#584e51"
+
+background = "#0c0b0c"
+dark_background = "#090809"
+darker_background = "#060606"
+lighter_background = "#0c0b0c"
+
+foreground = "#FAFCFB"
+dark_foreground = "#584e51"
+light_foreground = "#cfd3cd"
+bright_foreground = "#e2dddc"
+
 hyprland_active_border = "rgba(8a8588ee) rgba(e2dddcee)"
 hyprland_inactive_border = "rgba(584e51aa)"
 active_border_color = "#d6d3de"
 active_tab_background = "#a5a0b6"
 
-bg = "#0c0b0c"
-dark_bg = "#090809"
-darker_bg = "#060606"
-lighter_bg = "#0c0b0c"
-selection = "#584e51"
-muted = "#584e51"
-dark_fg = "#584e51"
-fg = "#FAFCFB"
-light_fg = "#cfd3cd"
-bright_fg = "#e2dddc"
-
 red = "#c38b7b"
 yellow = "#6B5E73"
 green = "#87a9b0"