Update app.css and ViewportCanvas.tsx for WebGL fallback handling

src/app/app.css

@@ -290,6 +290,27 @@ button:disabled {
   height: 100%;
 }
 
+.viewport-canvas__fallback {
+  position: absolute;
+  inset: 18px;
+  display: flex;
+  flex-direction: column;
+  justify-content: flex-end;
+  gap: 8px;
+  padding: 18px;
+  color: #f3e8da;
+  background: linear-gradient(180deg, rgba(8, 10, 14, 0.12) 0%, rgba(8, 10, 14, 0.58) 100%);
+  border: 1px solid rgba(255, 255, 255, 0.12);
+  border-radius: 18px;
+}
+
+.viewport-canvas__fallback-title {
+  font-size: 0.78rem;
+  font-weight: 700;
+  letter-spacing: 0.14em;
+  text-transform: uppercase;
+}
+
 .status-bar {
   display: flex;
   align-items: center;

src/viewport-three/ViewportCanvas.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useRef } from "react";
+import { useEffect, useRef, useState } from "react";
 
 import type { WorldSettings } from "../document/scene-document";
 
@@ -11,6 +11,7 @@ interface ViewportCanvasProps {
 export function ViewportCanvas({ world }: ViewportCanvasProps) {
   const containerRef = useRef<HTMLDivElement | null>(null);
   const hostRef = useRef<ViewportHost | null>(null);
+  const [viewportMessage, setViewportMessage] = useState<string | null>(null);
 
   useEffect(() => {
     const container = containerRef.current;
@@ -19,19 +20,46 @@ export function ViewportCanvas({ world }: ViewportCanvasProps) {
       return;
     }
 
-    const viewportHost = new ViewportHost();
-    hostRef.current = viewportHost;
-    viewportHost.mount(container);
+    const testCanvas = document.createElement("canvas");
+    const hasWebGl =
+      testCanvas.getContext("webgl2") !== null ||
+      testCanvas.getContext("webgl") !== null ||
+      testCanvas.getContext("experimental-webgl") !== null;
 
-    return () => {
-      viewportHost.dispose();
-      hostRef.current = null;
-    };
+    if (!hasWebGl) {
+      setViewportMessage("WebGL is unavailable in this browser environment. The viewport shell is visible, but rendering is disabled.");
+      return;
+    }
+
+    try {
+      const viewportHost = new ViewportHost();
+      hostRef.current = viewportHost;
+      viewportHost.mount(container);
+      setViewportMessage(null);
+
+      return () => {
+        viewportHost.dispose();
+        hostRef.current = null;
+      };
+    } catch (error) {
+      const message = error instanceof Error ? error.message : "Viewport initialization failed.";
+      setViewportMessage(`Viewport initialization failed: ${message}`);
+      return;
+    }
   }, []);
 
   useEffect(() => {
     hostRef.current?.updateWorld(world);
   }, [world]);
 
-  return <div ref={containerRef} className="viewport-canvas" data-testid="viewport-shell" aria-label="Editor viewport" />;
+  return (
+    <div ref={containerRef} className="viewport-canvas" data-testid="viewport-shell" aria-label="Editor viewport">
+      {viewportMessage === null ? null : (
+        <div className="viewport-canvas__fallback" role="status">
+          <div className="viewport-canvas__fallback-title">Viewport Unavailable</div>
+          <div>{viewportMessage}</div>
+        </div>
+      )}
+    </div>
+  );
 }