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/kitty.conf.tpl

@@ -1,22 +1,22 @@
-foreground {{ fg }}
-background {{ bg }}
+foreground {{ foreground }}
+background {{ background }}
 selection_foreground {{ selection_foreground }}
 selection_background {{ selection_background }}
 
-cursor {{ bright_fg }}
-cursor_text_color {{ bg }}
+cursor {{ bright_foreground }}
+cursor_text_color {{ background }}
 
 active_border_color {{ accent }}
 active_tab_background {{ accent }}
 
-color0 {{ bg }}
+color0 {{ background }}
 color1 {{ red }}
 color2 {{ green }}
 color3 {{ yellow }}
 color4 {{ blue }}
 color5 {{ magenta }}
 color6 {{ cyan }}
-color7 {{ fg }}
+color7 {{ foreground }}
 color8 {{ muted }}
 color9 {{ bright_red }}
 color10 {{ bright_green }}
@@ -24,4 +24,4 @@ color11 {{ bright_yellow }}
 color12 {{ bright_blue }}
 color13 {{ bright_magenta }}
 color14 {{ bright_cyan }}
-color15 {{ bright_fg }}
+color15 {{ bright_foreground }}