replace day/night cycle with a binary light/dark mode tied to UserSettings

The cycling sun/moon animation was distracting and not a fan favorite.
Drops the cycle path entirely — RenderSettings.dayNight.mode is now
"light" | "dark", and the cycle-only fields (cycleTicks, startPhase,
noonHold, nightHold) plus the passEnabled.dayNight toggle are gone.
getAmbient is a one-liner. The in-game mode follows the existing
darkMode UserSetting (same one that drives the page-level CSS class);
ClientGameRunner applies it on startup and on the per-key change event.

src/client/render/gl/render-settings.json

@@ -9,7 +9,6 @@
     "railroad": true,
     "fx": true,
     "bar": true,
-    "dayNight": true,
     "nameDebug": false
   },
   "falloutBloom": {
@@ -34,11 +33,7 @@
     "heatDecayPerTick": 1
   },
   "dayNight": {
-    "mode": "cycle",
-    "cycleTicks": 6000,
-    "startPhase": 0,
-    "noonHold": 0.25,
-    "nightHold": 0.1,
+    "mode": "light",
     "nightAmbient": 0.15,
     "dayAmbient": 1,
     "falloffPower": 2,