Add ocean color override to graphics settings (#4269)

## What

Adds a **Terrain** section to the graphics settings modal with a color
picker and a hex-code text field (paste a `#rrggbb` code) for the
**ocean** (deep water) color.

## Details

- The picked color sets the *shallow-water base*; the existing per-depth
brightness gradient is preserved (deeper water still darkens).
- Only deep water is affected — shoreline water and land are untouched.
- Follows the same override pattern as every other graphics setting: the
default lives in `render-settings.json` (`terrain.oceanColor`), the
override is a field in `GraphicsOverrides`, and `applyGraphicsOverrides`
copies it into the live `RenderSettings`.
- Rebased on #4271 (settings resolved before renderer construction): the
terrain texture **bakes the resolved ocean color at construction**, so a
saved override shows on load with no special-casing. Terrain is baked
into a GPU texture rather than read per-frame, so a *live* change still
triggers an explicit `view.rebuildTerrain()`.
- Resetting graphics overrides clears it back to the default ocean
color.

## Testing
Verified live in a headless singleplayer game:
- A **saved** ocean override renders green deep-water on load, baked at
construction with no settings-change event fired.
- A mid-game color change recolors the deep ocean instantly, gradient
preserved, shoreline/land untouched.

`tsc` and ESLint clean.

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

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

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

@@ -15,6 +15,9 @@
     "bar": true,
     "nameDebug": false
   },
+  "terrain": {
+    "oceanColor": "#4785b5"
+  },
   "falloutBloom": {
     "broilSpeedCold": 0.0018,
     "broilSpeedHot": 0,