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>

resources/lang/en.json

@@ -537,6 +537,10 @@
     "hover_glow_width_label": "Hover glow size",
     "icon_size_desc": "How large structure icons are drawn on the map",
     "icon_size_label": "Structure icon size",
+    "lighting_ambient_desc": "Darkens the map and adds a glow around structures (0 = off)",
+    "lighting_ambient_label": "Ambient light",
+    "lighting_unit_glow_desc": "How far the glow spreads around units and structures",
+    "lighting_unit_glow_label": "Unit glow",
     "name_cull_desc": "Hide names smaller than this size",
     "name_cull_label": "Minimum name size",
     "name_scale_label": "Name Scale",
@@ -550,6 +554,7 @@
     "reset_label": "Reset to defaults",
     "section_accessibility": "Accessibility",
     "section_effects": "Effects",
+    "section_lighting": "Lighting",
     "section_map": "Map",
     "section_name_labels": "Name Labels",
     "section_structure_icons": "Structure Icons",
@@ -1345,8 +1350,6 @@
     "coordinate_grid_label": "Coordinate Grid",
     "cursor_cost_label_desc": "Show a cost pill under the build cursor icon",
     "cursor_cost_label_label": "Cursor Build Cost",
-    "dark_mode_desc": "Toggle the site’s appearance between light and dark themes",
-    "dark_mode_label": "Dark Mode",
     "development_only": "Development Only",
     "easter_bug_count_desc": "How many bugs you're okay with (0–1000, emotionally)",
     "easter_bug_count_label": "Bug Count",