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/ethereal/colors.toml

@@ -1,17 +1,18 @@
 mode = "dark"
 
 accent = "#7d82d9"
-
-bg = "#060B1E"
-dark_bg = "#040816"
-darker_bg = "#030610"
-lighter_bg = "#131a3a"
 selection = "#252e56"
 muted = "#3d4573"
-dark_fg = "#6d7db6"
-fg = "#ffcead"
-light_fg = "#c9b8a6"
-bright_fg = "#ffcead"
+
+background = "#060B1E"
+dark_background = "#040816"
+darker_background = "#030610"
+lighter_background = "#131a3a"
+
+foreground = "#ffcead"
+dark_foreground = "#6d7db6"
+light_foreground = "#c9b8a6"
+bright_foreground = "#ffcead"
 
 red = "#ED5B5A"
 yellow = "#E9BB4F"