Move special effects toggle to graphics settings, wire to renderer

The special effects toggle wrote settings.specialEffects but nothing in
the WebGL pipeline read it — the FX pass is gated on passEnabled.fx. The
setting was orphaned when the old canvas renderer was removed, so the
toggle had no visual effect.

Move the toggle into the graphics settings modal (under a new Effects
section) and remove it from the in-game settings modal and the homepage
user settings modal. Rewire it to a passEnabled.fx graphics override so
it actually toggles the FX pass, applied live via the existing graphics
override listener.

Delete the now-dead fxLayer()/toggleFxLayer() from UserSettings.

Note: users who previously disabled special effects will reset to on,
since the old settings.specialEffects key is no longer read.

resources/lang/en.json

@@ -946,6 +946,7 @@
     "highlight_thicken_desc": "How much the border thickens on hover",
     "rail_distance_label": "Train track draw distance",
     "rail_distance_desc": "How far zoomed out train tracks remain visible",
+    "section_effects": "Effects",
     "reset_label": "Reset to defaults",
     "reset_desc": "Clear all graphics overrides"
   },