WebNN support initial

src/worker/inference.worker.ts

@@ -86,14 +86,22 @@ function forwardProgress(info: ProgressInfo): void {
 // Device selection (D-004): WebGPU when available, automatic WASM fallback
 // ---------------------------------------------------------------------------
 
-interface NavigatorWithGpu {
+interface NavigatorWithHardware {
   gpu?: { requestAdapter(): Promise<unknown | null> };
+  ml?: unknown;
 }
 
 async function detectDevice(preference: DevicePreference): Promise<InferenceDevice> {
   if (preference === "wasm") return "wasm";
+  if (preference === "webnn") {
+    try {
+      return (navigator as NavigatorWithHardware).ml ? "webnn" : "wasm";
+    } catch {
+      return "wasm";
+    }
+  }
   try {
-    const adapter = await (navigator as NavigatorWithGpu).gpu?.requestAdapter();
+    const adapter = await (navigator as NavigatorWithHardware).gpu?.requestAdapter();
     return adapter ? "webgpu" : "wasm";
   } catch {
     return "wasm";
@@ -127,7 +135,8 @@ async function createPipeline(
   // If WebGPU was selected but pipeline creation throws (driver quirks,
   // unsupported ops, out-of-memory), fall back to WASM automatically (D-004).
   const attempts: InferenceDevice[] =
-    detected === "webgpu" ? ["webgpu", "wasm"] : ["wasm"];
+    detected === "webgpu" ? ["webgpu", "wasm"] :
+    detected === "webnn" ? ["webnn", "wasm"] : ["wasm"];
 
   let lastError: unknown;
   for (const device of attempts) {