Replace dark mode with player-adjustable lighting (#4280)

## What

Removes the binary **dark mode** feature and replaces it with a
player-adjustable **Lighting** section in graphics settings.

### In-game settings
- Removed the Dark Mode toggle from both `SettingsModal` and
`UserSettingModal`, and `darkMode()`/`toggleDarkMode()`/`DARK_MODE_KEY`
from `UserSettings`.

### New Lighting section (Graphics Settings)
- **Ambient light** slider (1–3): mapped to the renderer's ambient as
`ambient = 1 / level`. **1.0 = no effect (unchanged look), 3.0 = darkest
with the strongest structure glow.**
- **Light falloff** slider (1–3): writes straight to
`lighting.falloffPower`.
- Lighting auto-enables only when ambient < 1, so the default (slider at
1) has zero GPU cost — off by default.

### Removed dark-mode overrides
- Deleted `applyDarkModeOverride()` + `DARK_AMBIENT` and their wiring in
`ClientGameRunner`, `gl/index.ts`, and the `DARK_MODE_KEY` listener.
- Removed the `.dark` HUD-class toggle in `Main.ts` and the
`userSettings.darkMode()` read in `PlayerIcons`.

### Train glow
- `UT_TRAIN` light reduced (intensity `2.0 → 0.5`, radius `8 → 6`) so
structures dominate the glow.

## Notes
- Removing the dark-mode setting also retires the HUD's Tailwind dark
theme (same setting). The dormant `dark:` CSS variants and unused
white-icon assets are left in place (out of scope).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/GraphicsOverrides.test.ts

@@ -74,6 +74,19 @@ describe("GraphicsOverridesSchema", () => {
     }
   });
 
+  test("accepts partial lighting overrides", () => {
+    const cases = [
+      { lighting: {} },
+      { lighting: { ambient: 0.5 } },
+      { lighting: { ambient: 1 } },
+      { lighting: { falloffPower: 2 } },
+      { lighting: { ambient: 0.3, falloffPower: 1.5 } },
+    ];
+    for (const c of cases) {
+      expect(GraphicsOverridesSchema.safeParse(c).success).toBe(true);
+    }
+  });
+
   test("rejects wrong field types", () => {
     expect(
       GraphicsOverridesSchema.safeParse({ name: { nameScaleFactor: "big" } })
@@ -115,6 +128,16 @@ describe("GraphicsOverridesSchema", () => {
         railroad: { railThickness: "wide" },
       }).success,
     ).toBe(false);
+    expect(
+      GraphicsOverridesSchema.safeParse({
+        lighting: { ambient: "dark" },
+      }).success,
+    ).toBe(false);
+    expect(
+      GraphicsOverridesSchema.safeParse({
+        lighting: { falloffPower: "soft" },
+      }).success,
+    ).toBe(false);
   });
 });
 
@@ -351,6 +374,55 @@ describe("applyGraphicsOverrides", () => {
     expect(z.railThickness).toBe(defaults.railThickness);
   });
 
+  test("ambient < 1 sets ambient and enables the lighting pass", () => {
+    const l = gen({ lighting: { ambient: 0.5 } }).lighting;
+    expect(l.ambient).toBe(0.5);
+    expect(l.enabled).toBe(true);
+  });
+
+  test("ambient === 1 sets ambient but leaves lighting disabled (identity)", () => {
+    const l = gen({ lighting: { ambient: 1 } }).lighting;
+    expect(l.ambient).toBe(1);
+    expect(l.enabled).toBe(false);
+  });
+
+  test("ambient absent → lighting stays at render-settings.json defaults", () => {
+    const defaults = createRenderSettings().lighting;
+    expect(gen({}).lighting.ambient).toBe(defaults.ambient);
+    expect(gen({}).lighting.enabled).toBe(defaults.enabled);
+    expect(gen({ lighting: {} }).lighting.enabled).toBe(defaults.enabled);
+  });
+
+  test("applies falloffPower override (including values below default)", () => {
+    expect(gen({ lighting: { falloffPower: 1.4 } }).lighting.falloffPower).toBe(
+      1.4,
+    );
+    expect(gen({ lighting: { falloffPower: 3 } }).lighting.falloffPower).toBe(
+      3,
+    );
+  });
+
+  test("falloffPower override alone does not enable the lighting pass", () => {
+    expect(gen({ lighting: { falloffPower: 1.4 } }).lighting.enabled).toBe(
+      false,
+    );
+  });
+
+  test("lighting override leaves other lighting fields at defaults", () => {
+    const defaults = createRenderSettings().lighting;
+    const l = gen({ lighting: { ambient: 0.4 } }).lighting;
+    expect(l.falloffPower).toBe(defaults.falloffPower);
+    expect(l.blurZoomDivisor).toBe(defaults.blurZoomDivisor);
+    expect(l.lightRadiusMultiplier).toBe(defaults.lightRadiusMultiplier);
+  });
+
+  test("ambient + falloffPower compose together", () => {
+    const l = gen({ lighting: { ambient: 0.3, falloffPower: 1 } }).lighting;
+    expect(l.ambient).toBe(0.3);
+    expect(l.falloffPower).toBe(1);
+    expect(l.enabled).toBe(true);
+  });
+
   test("classicIcons + name overrides compose independently", () => {
     const s = gen({
       name: { darkNames: true, nameScaleFactor: 0.9 },