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/ghostty.conf.tpl

@@ -1,17 +1,17 @@
-background = {{ bg }}
-foreground = {{ fg }}
-cursor-color = {{ bright_fg }}
+background = {{ background }}
+foreground = {{ foreground }}
+cursor-color = {{ bright_foreground }}
 selection-background = {{ selection_background }}
 selection-foreground = {{ selection_foreground }}
 
-palette = 0={{ bg }}
+palette = 0={{ background }}
 palette = 1={{ red }}
 palette = 2={{ green }}
 palette = 3={{ yellow }}
 palette = 4={{ blue }}
 palette = 5={{ magenta }}
 palette = 6={{ cyan }}
-palette = 7={{ fg }}
+palette = 7={{ foreground }}
 palette = 8={{ muted }}
 palette = 9={{ bright_red }}
 palette = 10={{ bright_green }}
@@ -19,4 +19,4 @@ palette = 11={{ bright_yellow }}
 palette = 12={{ bright_blue }}
 palette = 13={{ bright_magenta }}
 palette = 14={{ bright_cyan }}
-palette = 15={{ bright_fg }}
+palette = 15={{ bright_foreground }}