Adds a 'WebGL Renderer' toggle to Settings > Appearance (desktop). WebGL
stays on by default; users can turn it off to force the DOM renderer when
they hit GPU glitches, without needing the ?nowebgl URL param. Explicit
opt-in (or ?webgl=force) clears a stale auto-fallback marker. Mobile skip
and the long-task auto-fallback safety net are unchanged.
The device/param/sticky/pref interaction is factored into a pure,
unit-tested shouldSkipWebGL() helper in constants.js.
