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/helix.toml.tpl

@@ -116,17 +116,17 @@ info = "color4"
 hint = "color6"
 
 [palette]
-background = "{{ bg }}"
-foreground = "{{ fg }}"
-cursor = "{{ bright_fg }}"
+background = "{{ background }}"
+foreground = "{{ foreground }}"
+cursor = "{{ bright_foreground }}"
 selection_background = "{{ selection_background }}"
 selection_foreground = "{{ selection_foreground }}"
-color0 = "{{ bg }}"
+color0 = "{{ background }}"
 color1 = "{{ red }}"
 color2 = "{{ green }}"
 color3 = "{{ yellow }}"
 color4 = "{{ blue }}"
 color5 = "{{ magenta }}"
 color6 = "{{ cyan }}"
-color7 = "{{ fg }}"
+color7 = "{{ foreground }}"
 color8 = "{{ muted }}"