web: add loading screen while WASM fetches

Shows a themed spinner overlay while the large WASM bundle downloads,
improving perceived load time on slower connections.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

gui/Dioxus.toml

@@ -23,7 +23,7 @@ watch_path = ["src", "assets"]
 # CSS style file
 style = []
 # Javascript code file
-script = []
+script = ["loader.js"]
 
 [web.resource.dev]
 # serve: [dev-server] only

gui/public/loader.js

@@ -0,0 +1,65 @@
+// Loading screen that displays while WASM loads
+(function() {
+  // Create and inject loader styles immediately (head exists)
+  var style = document.createElement('style');
+  style.textContent =
+    '.wasm-loader {' +
+      'position: fixed;' +
+      'top: 0;' +
+      'left: 0;' +
+      'width: 100%;' +
+      'height: 100%;' +
+      'background-color: oklch(0.15 0.01 338.64);' +
+      'display: flex;' +
+      'align-items: center;' +
+      'justify-content: center;' +
+      'z-index: 9999;' +
+      'transition: opacity 0.3s ease-out;' +
+    '}' +
+    '.wasm-loader.hidden {' +
+      'opacity: 0;' +
+      'pointer-events: none;' +
+    '}' +
+    '.wasm-spinner {' +
+      'width: 48px;' +
+      'height: 48px;' +
+      'border: 4px solid rgba(123, 173, 159, 0.2);' +
+      'border-top-color: #7bad9f;' +
+      'border-radius: 50%;' +
+      'animation: wasm-spin 1s linear infinite;' +
+    '}' +
+    '@keyframes wasm-spin {' +
+      'to { transform: rotate(360deg); }' +
+    '}';
+  document.head.appendChild(style);
+
+  function init() {
+    // Create loader element
+    var loader = document.createElement('div');
+    loader.className = 'wasm-loader';
+    loader.innerHTML = '<div class="wasm-spinner"></div>';
+    document.body.appendChild(loader);
+
+    // Watch for Dioxus to mount content in #main
+    var observer = new MutationObserver(function(mutations, obs) {
+      var main = document.getElementById('main');
+      if (main && main.children.length > 0) {
+        loader.classList.add('hidden');
+        setTimeout(function() { loader.remove(); }, 300);
+        obs.disconnect();
+      }
+    });
+
+    observer.observe(document.body, {
+      childList: true,
+      subtree: true
+    });
+  }
+
+  // Wait for body to exist
+  if (document.body) {
+    init();
+  } else {
+    document.addEventListener('DOMContentLoaded', init);
+  }
+})();