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.

functions/__config_toggle_apply.fish

@@ -0,0 +1,53 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   __config_toggle_apply <varname> <scope> <value>
+#
+# DESCRIPTION
+#   Applies a state value to an opinionated-component variable in the given
+#   scope, immediately and persistently. Used by config-toggle's Space (cycle)
+#   and Left/Right (directional) handlers so the set/erase logic lives in one
+#   place. A value of "DEFAULT" erases the variable in that scope so the
+#   master switch / built-in default takes over again.
+#
+#     universal → set -U  (on/off)  /  set -Ue  (DEFAULT)
+#     session   → set -g  (on/off)  /  set -eg  (DEFAULT)
+#
+# ARGUMENTS
+#   varname  Variable name without the $ prefix
+#   scope    "universal" or "session"
+#   value    "on", "off", or "DEFAULT"
+#
+# RETURNS
+#   0  Always
+#
+# EXAMPLE
+#   __config_toggle_apply __fish_config_op_aliases universal off
+#   __config_toggle_apply __fish_config_op_greeting session DEFAULT
+function __config_toggle_apply
+    set -l varname $argv[1]
+    set -l scope   $argv[2]
+    set -l value   $argv[3]
+
+    switch $scope
+        case universal
+            switch $value
+                case on
+                    set -U $varname on
+                case off
+                    set -U $varname off
+                case DEFAULT
+                    set -Ue $varname
+            end
+        case session
+            switch $value
+                case on
+                    set -g $varname on
+                case off
+                    set -g $varname off
+                case DEFAULT
+                    set -eg $varname
+            end
+    end
+end