docs: Update README with instructions for building and serving Flutter web release

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

apps/wolf_3d_gui/README.md

@@ -75,6 +75,22 @@ Game data directory selection/persistence is managed by app managers and Flutter
 - **No game data discovered**: choose a valid directory in the picker and ensure files are present.
 - **Linux build/runtime issues**: verify native dependency packages are installed.
 - **Web target limitations**: use desktop target for full native-audio/path behavior.
+- **Web release build won’t load when opening `index.html` directly**: Flutter web output must be served over `http://` or `https://`, not opened with `file://`.
+
+ Build and serve locally:
+
+ ```bash
+ flutter build web --release
+ python3 -m http.server 8080 -d build/web
+ ```
+
+ Then open `http://localhost:8080` in your browser.
+
+ If deploying under a subpath, build with matching base href, for example:
+
+ ```bash
+ flutter build web --release --base-href /wolf_dart/
+ ```
 
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