feat(config-toggle): add Left/Right arrows for directional value changes

Left/Right now adjust the highlighted value one step along the
OFF ← DEFAULT → ON scale, clamped at the ends (no wrap), complementing
Space which cycles through all states and wraps. Right steps toward ON,
Left toward OFF.

Extract the set/erase side-effect into __config_toggle_apply so Space,
Left, and Right share one implementation of the scope-aware
set -U/-g/-Ue/-eg logic. Update the in-panel keybind hint (now
width-padded via string pad so the border stays aligned) and the help
text and offline docs to cover the new keys.

docs/fish-config.md

@@ -1232,15 +1232,20 @@ Add -i (interactive confirmation) to destructive commands:
       Universal — persists across all sessions (set -U)
       Session   — current shell only (set -g)
 
-    Changes apply immediately on each Space keypress. Always available
+    Changes apply immediately on each value keypress. Always available
     regardless of the __fish_config_opinionated master state.
 
     Navigation:
       ↑ ↓ / j k   Move cursor
-      Space        Cycle: ON → OFF → DEFAULT → ON
+      ← →          Set value directionally: OFF ← DEFAULT → ON (clamped)
+      Space        Cycle through all states: ON → OFF → DEFAULT → ON (wraps)
       Tab          Switch scope (Universal ↔ Session)
       q / Escape   Exit
 
+    Left/Right move one step along the OFF–DEFAULT–ON scale and stop at the
+    ends; Space cycles through every state and wraps around. DEFAULT erases
+    the variable so the master switch / built-in default applies.
+
     Flags:
       --help / -h   Show usage.