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>

default/themed/foot.ini.tpl

@@ -1,19 +1,19 @@
 [colors-dark]
-foreground={{ fg_strip }}
-background={{ bg_strip }}
+foreground={{ foreground_strip }}
+background={{ background_strip }}
 selection-foreground={{ selection_foreground_strip }}
 selection-background={{ selection_background_strip }}
 
-cursor={{ bg_strip }} {{ bright_fg_strip }}
+cursor={{ background_strip }} {{ bright_foreground_strip }}
 
-regular0={{ bg_strip }}
+regular0={{ background_strip }}
 regular1={{ red_strip }}
 regular2={{ green_strip }}
 regular3={{ yellow_strip }}
 regular4={{ blue_strip }}
 regular5={{ purple_strip }}
 regular6={{ cyan_strip }}
-regular7={{ fg_strip }}
+regular7={{ foreground_strip }}
 
 bright0={{ muted_strip }}
 bright1={{ bright_red_strip }}
@@ -22,4 +22,4 @@ bright3={{ bright_yellow_strip }}
 bright4={{ bright_blue_strip }}
 bright5={{ bright_magenta_strip }}
 bright6={{ bright_cyan_strip }}
-bright7={{ bright_fg_strip }}
+bright7={{ bright_foreground_strip }}