feat: fix CompactSeedQR binary QR code scanning with jsQR library

- Replace BarcodeDetector with jsQR for raw binary byte access
- BarcodeDetector forced UTF-8 decoding which corrupted binary data
- jsQR's binaryData property preserves raw bytes without text conversion
- Fix regex bug: use single backslash \x00 instead of \x00 for binary detection
- Add debug logging for scan data inspection
- QR generation already worked (Krux-compatible), only scanning was broken

Resolves binary QR code scanning for 12/24-word CompactSeedQR format.
Tested with Krux device - full bidirectional compatibility confirmed.

src/lib/seedqr.ts

@@ -0,0 +1,111 @@
+/**
+ * @file seedqr.ts
+ * @summary Implements encoding and decoding for Seedsigner's SeedQR format.
+ * @description This module provides functions to convert BIP39 mnemonics to and from the
+ * SeedQR format, supporting both the Standard (numeric) and Compact (hex) variations.
+ * The logic is adapted from the official Seedsigner specification and test vectors.
+ */
+
+import { BIP39_WORDLIST, WORD_INDEX, mnemonicToEntropy, entropyToMnemonic } from './seedblend';
+
+// Helper to convert a hex string to a Uint8Array in a browser-compatible way.
+function hexToUint8Array(hex: string): Uint8Array {
+  if (hex.length % 2 !== 0) {
+    throw new Error('Hex string must have an even number of characters');
+  }
+  const bytes = new Uint8Array(hex.length / 2);
+  for (let i = 0; i < bytes.length; i++) {
+    bytes[i] = parseInt(hex.substr(i * 2, 2), 16);
+  }
+  return bytes;
+}
+
+/**
+ * Decodes a Standard SeedQR (numeric digit stream) into a mnemonic phrase.
+ * @param digitStream A string containing 4-digit numbers representing BIP39 word indices.
+ * @returns The decoded BIP39 mnemonic.
+ */
+function decodeStandardSeedQR(digitStream: string): string {
+  if (digitStream.length % 4 !== 0) {
+    throw new Error('Invalid Standard SeedQR: Length must be a multiple of 4.');
+  }
+
+  const wordIndices: number[] = [];
+  for (let i = 0; i < digitStream.length; i += 4) {
+    const indexStr = digitStream.slice(i, i + 4);
+    const index = parseInt(indexStr, 10);
+    if (isNaN(index) || index >= 2048) {
+      throw new Error(`Invalid word index in SeedQR: ${indexStr}`);
+    }
+    wordIndices.push(index);
+  }
+
+  if (wordIndices.length !== 12 && wordIndices.length !== 24) {
+    throw new Error(`Invalid word count from SeedQR: ${wordIndices.length}. Must be 12 or 24.`);
+  }
+
+  const mnemonicWords = wordIndices.map(index => BIP39_WORDLIST[index]);
+  return mnemonicWords.join(' ');
+}
+
+/**
+ * Decodes a Compact SeedQR (hex-encoded entropy) into a mnemonic phrase.
+ * @param hexEntropy The hex-encoded entropy string.
+ * @returns A promise that resolves to the decoded BIP39 mnemonic.
+ */
+async function decodeCompactSeedQR(hexEntropy: string): Promise<string> {
+  const entropy = hexToUint8Array(hexEntropy);
+  if (entropy.length !== 16 && entropy.length !== 32) {
+    throw new Error(`Invalid entropy length for Compact SeedQR: ${entropy.length}. Must be 16 or 32 bytes.`);
+  }
+  return entropyToMnemonic(entropy);
+}
+
+/**
+ * A unified decoder that automatically detects and parses a SeedQR string.
+ * @param qrData The raw data from the QR code.
+ * @returns A promise that resolves to the decoded BIP39 mnemonic.
+ */
+export async function decodeSeedQR(qrData: string): Promise<string> {
+  const trimmed = qrData.trim();
+  // Standard SeedQR is a string of only digits.
+  if (/^\d+$/.test(trimmed)) {
+    return decodeStandardSeedQR(trimmed);
+  }
+  // Compact SeedQR is a hex string.
+  if (/^[0-9a-fA-F]+$/.test(trimmed)) {
+    return decodeCompactSeedQR(trimmed);
+  }
+  throw new Error('Unsupported or invalid SeedQR format.');
+}
+
+/**
+ * Encodes a mnemonic into the Standard SeedQR format (numeric digit stream).
+ * @param mnemonic The BIP39 mnemonic string.
+ * @returns A promise that resolves to the Standard SeedQR string.
+ */
+export async function encodeStandardSeedQR(mnemonic: string): Promise<string> {
+    const words = mnemonic.trim().toLowerCase().split(/\s+/);
+    if (words.length !== 12 && words.length !== 24) {
+        throw new Error("Mnemonic must be 12 or 24 words to generate a SeedQR.");
+    }
+    
+    const digitStream = words.map(word => {
+        const index = WORD_INDEX.get(word);
+        if (index === undefined) {
+            throw new Error(`Invalid word in mnemonic: ${word}`);
+        }
+        return index.toString().padStart(4, '0');
+    }).join('');
+
+    return digitStream;
+}
+
+/**
+ * Encodes a mnemonic into the Compact SeedQR format (raw entropy bytes).
+ * @param mnemonic The BIP39 mnemonic string.
+ * @returns A promise that resolves to the Compact SeedQR entropy as a Uint8Array.
+ */
+export async function encodeCompactSeedQREntropy(mnemonic: string): Promise<Uint8Array> {
+    return await mnemonicToEntropy(mnemonic);
+}