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>

src/client/UserSettingModal.ts

@@ -200,12 +200,6 @@ export class UserSettingModal extends BaseModal {
     }, 5000);
   }
 
-  toggleDarkMode() {
-    this.userSettings.toggleDarkMode();
-
-    console.log("🌙 Dark Mode:", this.userSettings.darkMode() ? "ON" : "OFF");
-  }
-
   /** Whether colorblind mode is currently enabled in the graphics overrides. */
   private colorblindMode(): boolean {
     return (
@@ -752,15 +746,6 @@ export class UserSettingModal extends BaseModal {
 
   private renderBasicSettings() {
     return html`
-      <!-- 🌙 Dark Mode -->
-      <setting-toggle
-        label="${translateText("user_setting.dark_mode_label")}"
-        description="${translateText("user_setting.dark_mode_desc")}"
-        id="dark-mode-toggle"
-        .checked=${this.userSettings.darkMode()}
-        @change=${this.toggleDarkMode}
-      ></setting-toggle>
-
       <!-- 🎨 Colorblind Mode -->
       <setting-toggle
         label="${translateText("user_setting.colorblind_label")}"