fix(web): ship WGSL shaders to dist via trunk copy-dir

Bevy's HttpWasmAssetReader fetches asset paths over HTTP at runtime,
requesting 'assets/shaders/*.wgsl' (matching AssetPlugin's default
file_path of "assets"). Trunk only emits what index.html references,
so without an explicit copy-dir link the shaders were absent from dist/
and every material hit a 404 → grey screen.

Add the copy-dir link so trunk mirrors assets/ into dist/.

web/index.html

@@ -17,5 +17,11 @@
       data-cargo-features="bevy/webgpu"
       data-wasm-opt="z"
     />
+    <!-- Ship the WGSL shaders so Bevy's WasmAssetReader can fetch them at
+         runtime (it requests `assets/shaders/*.wgsl` over HTTP, matching the
+         default AssetPlugin file_path of "assets"). Without this the shaders
+         are absent from dist/ and every material loads a 404 fallback → grey
+         screen. -->
+    <link data-trunk rel="copy-dir" href="../assets" />
   </body>
 </html>