fix(blender): correct isomorphic crypto loading

Refactors the crypto module loading in `seedblend.ts` to be truly isomorphic and prevent browser runtime errors.

- Replaces the static Node.js `crypto` import with a dynamic `import()` inside a singleton promise (`getCrypto`).
- This ensures Vite does not externalize the module for browser builds, resolving the 'Cannot access \'crypto.webcrypto\' in client code' error.
- The browser will use its native `window.crypto`, while the Node.js test environment dynamically loads the `crypto` module.
- All tests continue to pass, verifying the fix.

src/lib/seedblend.ts

@@ -23,16 +23,28 @@
  */
 
 import wordlistTxt from '../bip39_wordlist.txt?raw';
-import { webcrypto } from 'crypto';
 
 // --- Isomorphic Crypto Setup ---
 
-// Use browser crypto if available, otherwise fallback to Node.js webcrypto.
-// This allows the library to run in both the browser and the test environment (Node.js).
-const subtle = (typeof window !== 'undefined' && window.crypto?.subtle)
-  ? window.crypto.subtle
-  : webcrypto.subtle;
-
+let cryptoPromise: Promise<SubtleCrypto>;
+/**
+ * Asynchronously gets the appropriate SubtleCrypto interface, using a singleton
+ * pattern to ensure the module is loaded only once.
+ * This approach uses a dynamic import() to prevent Vite from bundling the
+ * Node.js 'crypto' module in browser builds.
+ */
+function getCrypto(): Promise<SubtleCrypto> {
+    if (!cryptoPromise) {
+        cryptoPromise = (async () => {
+            if (typeof window !== 'undefined' && window.crypto?.subtle) {
+                return window.crypto.subtle;
+            }
+            const { webcrypto } = await import('crypto');
+            return webcrypto.subtle;
+        })();
+    }
+    return cryptoPromise;
+}
 
 // --- BIP39 Wordlist Loading ---
 
@@ -61,6 +73,7 @@ if (BIP39_WORDLIST.length !== 2048) {
  * @returns A promise that resolves to the hash as a Uint8Array.
  */
 async function sha256(data: Uint8Array): Promise<Uint8Array> {
+  const subtle = await getCrypto();
   const hashBuffer = await subtle.digest('SHA-256', data);
   return new Uint8Array(hashBuffer);
 }
@@ -72,6 +85,7 @@ async function sha256(data: Uint8Array): Promise<Uint8Array> {
  * @returns A promise that resolves to the HMAC tag.
  */
 async function hmacSha256(key: Uint8Array, data: Uint8Array): Promise<Uint8Array> {
+    const subtle = await getCrypto();
     const cryptoKey = await subtle.importKey(
         'raw',
         key,