fix(web): ignore winit's control-flow exception on WASM init

The live demo showed "Failed to load WebAssembly bundle" with the
message "Using exceptions for control flow, don't mind me. This
isn't actually an error!".

That exception is winit's documented mechanism: on wasm32,
EventLoop::run() never returns, so it throws a JS exception to
unwind the Rust stack and hand control to the browser's event
loop. The app is fully running behind it — our try/catch was just
mis-classifying the benign unwind as a fatal error.

index.html now filters that specific exception (matches "control
flow" / "isn't actually an error") and reveals the canvas instead
of showing the error overlay. Genuine init failures still surface.

No WASM rebuild needed — pure JS-side fix.

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

Author: stevekwon211

web/index.html

@@ -60,19 +60,35 @@
 
     <script type="module">
         import init from './pkg/slintcn-showcase.js';
-        try {
-            await init();
-            // ui.run() returned (registers handlers, browser drives the loop).
-            // Hide the loader so Slint's canvas is visible.
+
+        function reveal() {
             document.getElementById('loader').classList.add('hidden');
-        } catch (err) {
-            const loader = document.getElementById('loader');
-            loader.innerHTML = `
-                <div class="brand">slintcn</div>
-                <div class="err">Failed to load WebAssembly bundle.\n${err && err.stack || err}</div>
-            `;
-            console.error('slintcn init failed:', err);
         }
+
+        function isWinitControlFlow(err) {
+            // winit hands control to the browser from EventLoop::run() by
+            // THROWING a JS exception that unwinds the Rust stack. It's not a
+            // failure — the app is already running behind it. The message is
+            // winit's own: "Using exceptions for control flow…".
+            const msg = (err && (err.message || err.toString())) || '';
+            return msg.includes('control flow') || msg.includes("isn't actually an error");
+        }
+
+        init()
+            .then(reveal)
+            .catch((err) => {
+                if (isWinitControlFlow(err)) {
+                    // Expected. The Slint canvas is live; just show it.
+                    reveal();
+                    return;
+                }
+                const loader = document.getElementById('loader');
+                loader.innerHTML = `
+                    <div class="brand">slintcn</div>
+                    <div class="err">Failed to load WebAssembly bundle.\n${(err && err.stack) || err}</div>
+                `;
+                console.error('slintcn init failed:', err);
+            });
     </script>
 </body>
 </html>