ci: Add WASM build + deploy workflow for live ImGui web demo

Add .github/workflows/deploy-wasm.yml that:
- Installs Emscripten SDK on ubuntu-latest
- Builds example_sdl2_opengl3 with Makefile.emscripten using -sSINGLE_FILE
  (all WASM+JS embedded in a single .html file, no separate .wasm/.js)
- Uses custom shell_imgui_xl.html with loading spinner and WebGL context
  loss handling
- Deploys to gh-pages/wasm/ directory via peaceiris/actions-gh-pages

The resulting live demo at turtle-pb.github.io/imgui_xl/wasm/index.html
will show the ACTUAL Dear ImGui demo window running in the browser via
WebAssembly — not a canvas simulation, but real compiled C++ ImGui.

Triggers on pushes to master that touch ImGui source files, the SDL2/
OpenGL3 backend, the example, or the shell template.
Also supports manual workflow_dispatch.

.github/workflows/deploy-wasm.yml

@@ -0,0 +1,63 @@
+name: Deploy ImGui WASM to GitHub Pages
+
+on:
+  push:
+    branches: [master]
+    paths:
+      - 'imgui.cpp'
+      - 'imgui.h'
+      - 'imgui_demo.cpp'
+      - 'imgui_draw.cpp'
+      - 'imgui_tables.cpp'
+      - 'imgui_widgets.cpp'
+      - 'backends/imgui_impl_sdl2.*'
+      - 'backends/imgui_impl_opengl3.*'
+      - 'examples/example_sdl2_opengl3/main.cpp'
+      - 'examples/example_sdl2_opengl3/Makefile.emscripten'
+      - 'examples/libs/emscripten/**'
+      - '.github/workflows/deploy-wasm.yml'
+  workflow_dispatch:
+
+permissions:
+  contents: write
+  pages: write
+
+jobs:
+  build-and-deploy:
+    runs-on: ubuntu-latest
+    name: Build ImGui WASM and deploy to gh-pages
+
+    steps:
+    - name: Checkout imgui_xl
+      uses: actions/checkout@v6
+
+    - name: Install Emscripten
+      run: |
+        wget -q https://github.com/emscripten-core/emsdk/archive/master.tar.gz
+        tar -xzf master.tar.gz
+        emsdk-master/emsdk update
+        emsdk-master/emsdk install latest
+        emsdk-master/emsdk activate latest
+        sudo apt-get install build-essential
+
+    - name: Build SDL2+OpenGL3 ImGui WASM
+      run: |
+        pushd emsdk-master
+        source ./emsdk_env.sh
+        popd
+
+        # Build with custom shell, single-file output (everything embedded in one .html)
+        make -C examples/example_sdl2_opengl3 -f Makefile.emscripten \
+          LDFLAGS+="-sSINGLE_FILE --shell-file ../libs/emscripten/shell_imgui_xl.html"
+
+        # Verify output
+        ls -la examples/example_sdl2_opengl3/web/
+
+    - name: Deploy to gh-pages
+      uses: peaceiris/actions-gh-pages@v4
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        publish_branch: gh-pages
+        publish_dir: examples/example_sdl2_opengl3/web
+        destination_dir: wasm
+        keep_files: true

examples/libs/emscripten/shell_imgui_xl.html

@@ -0,0 +1,62 @@
+<!doctype html>
+<html lang="en-us">
+  <head>
+    <meta charset="utf-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, minimum-scale=1, user-scalable=no"/>
+    <title>Dear ImGui — Live Web Demo (imgui_xl)</title>
+    <style>
+      body { margin: 0; background-color: #0d0e12; overflow: hidden; font-family: -apple-system, sans-serif; }
+      .emscripten {
+        position: absolute; top: 0; left: 0; margin: 0; border: 0;
+        width: 100%; height: 100%; overflow: hidden; display: block;
+      }
+      #canvas:focus { outline: none; }
+      #loading {
+        position: absolute; top: 50%; left: 50%; transform: translate(-50%, -50%);
+        text-align: center; color: #6b7080; font-size: 14px; z-index: 10;
+      }
+      #loading .spinner {
+        width: 40px; height: 40px; border: 3px solid #1c1f2b;
+        border-top-color: #4a9eff; border-radius: 50%;
+        animation: spin 1s linear infinite; margin: 0 auto 16px;
+      }
+      @keyframes spin { to { transform: rotate(360deg); } }
+      #loading h1 { color: #e8eaf0; font-size: 18px; margin: 0 0 8px; }
+      #loading p { margin: 0; }
+      #loading .hint { font-size: 12px; margin-top: 12px; color: #4a6fa5; }
+    </style>
+  </head>
+  <body>
+    <div id="loading">
+      <div class="spinner"></div>
+      <h1>Dear ImGui — Live Web Demo</h1>
+      <p>Loading WASM build...</p>
+      <p class="hint">Click the canvas to interact. Right-click works too.</p>
+    </div>
+    <canvas class="emscripten" id="canvas" oncontextmenu="event.preventDefault()" tabindex=-1></canvas>
+    <script type='text/javascript'>
+      var Module = {
+        preRun: [],
+        postRun: [],
+        print: function(text) { console.log(text); },
+        printErr: function(text) { console.error(text); },
+        canvas: (function() {
+          var canvas = document.getElementById('canvas');
+          canvas.addEventListener("webglcontextlost", function(e) {
+            alert('WebGL context lost. Please reload the page.');
+            e.preventDefault();
+          }, false);
+          return canvas;
+        })(),
+        onRuntimeInitialized: function() {
+          var loading = document.getElementById('loading');
+          if (loading) loading.style.display = 'none';
+        },
+        setStatus: function(text) { console.log("status: " + text); },
+        monitorRunDependencies: function(left) {}
+      };
+      window.onerror = function() { console.log("onerror: " + event); };
+    </script>
+    {{{ SCRIPT }}}
+  </body>
+</html>