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/tokyo-night/colors.toml

@@ -1,17 +1,18 @@
 mode = "dark"
 
 accent = "#7aa2f7"
-
-bg = "#1a1b26"
-dark_bg = "#13141c"
-darker_bg = "#0e0e14"
-lighter_bg = "#24283b"
 selection = "#292e42"
 muted = "#414868"
-dark_fg = "#565f89"
-fg = "#a9b1d6"
-light_fg = "#b4bee6"
-bright_fg = "#c0caf5"
+
+background = "#1a1b26"
+dark_background = "#13141c"
+darker_background = "#0e0e14"
+lighter_background = "#24283b"
+
+foreground = "#a9b1d6"
+dark_foreground = "#565f89"
+light_foreground = "#b4bee6"
+bright_foreground = "#c0caf5"
 
 red = "#f7768e"
 yellow = "#e0af68"