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/white/colors.toml

@@ -1,17 +1,18 @@
 mode = "light"
 
 accent = "#6e6e6e"
-
-bg = "#ffffff"
-dark_bg = "#bfbfbf"
-darker_bg = "#808080"
-lighter_bg = "#c0c0c0"
 selection = "#c0c0c0"
 muted = "#c0c0c0"
-dark_fg = "#c0c0c0"
-fg = "#000000"
-light_fg = "#000000"
-bright_fg = "#000000"
+
+background = "#ffffff"
+dark_background = "#bfbfbf"
+darker_background = "#808080"
+lighter_background = "#c0c0c0"
+
+foreground = "#000000"
+dark_foreground = "#c0c0c0"
+light_foreground = "#000000"
+bright_foreground = "#000000"
 
 red = "#2a2a2a"
 yellow = "#4a4a4a"