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/nord/colors.toml

@@ -1,17 +1,18 @@
 mode = "dark"
 
 accent = "#81a1c1"
-
-bg = "#2e3440"
-dark_bg = "#222730"
-darker_bg = "#191c23"
-lighter_bg = "#3b4252"
 selection = "#434c5e"
 muted = "#4c566a"
-dark_fg = "#667080"
-fg = "#d8dee9"
-light_fg = "#adb5c4"
-bright_fg = "#d8dee9"
+
+background = "#2e3440"
+dark_background = "#222730"
+darker_background = "#191c23"
+lighter_background = "#3b4252"
+
+foreground = "#d8dee9"
+dark_foreground = "#667080"
+light_foreground = "#adb5c4"
+bright_foreground = "#d8dee9"
 
 red = "#bf616a"
 yellow = "#ebcb8b"