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/everforest/colors.toml

@@ -1,17 +1,18 @@
 mode = "dark"
 
 accent = "#7fbbb3"
-
-bg = "#2d353b"
-dark_bg = "#21272c"
-darker_bg = "#181d20"
-lighter_bg = "#343f44"
 selection = "#3d484d"
 muted = "#475258"
-dark_fg = "#4f585e"
-fg = "#d3c6aa"
-light_fg = "#9da9a0"
-bright_fg = "#d3c6aa"
+
+background = "#2d353b"
+dark_background = "#21272c"
+darker_background = "#181d20"
+lighter_background = "#343f44"
+
+foreground = "#d3c6aa"
+dark_foreground = "#4f585e"
+light_foreground = "#9da9a0"
+bright_foreground = "#d3c6aa"
 
 red = "#e67e80"
 yellow = "#dbbc7f"