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_draw.fish

@@ -131,8 +131,10 @@ function __config_toggle_draw
     printf '│%s│\n' $HBR
 
     # ── Keybind hint ──────────────────────────────────────
-    printf '│  %s↑↓/jk: move  Space: cycle  Tab: scope  q: quit%s  │\n' \
-        $c_dim $c_reset
+    # Padded to the inner width so the right border stays aligned regardless
+    # of the hint text. string pad is width-aware (arrows count as 1 column).
+    set -l hint " ↑↓ move  ←→ set  Space cycle  Tab scope  q quit"
+    printf '│%s%s%s│\n' $c_dim (string pad -r -w $IW -- $hint) $c_reset
 
     # ── Bottom border ─────────────────────────────────────
     printf '└%s┘\n' $HBR