fix(qr): Revert to original 12-byte Amnezia header format

The working QR format from Amnezia app uses:
- 12-byte header: version (0x07C00100) + compressedLen + uncompressedLen
- zlib compressed JSON data

Previously I incorrectly changed this to Qt qCompress format (4-byte header).
This commit reverts to the correct Amnezia-compatible format.

inc/QrUtil.php

@@ -63,15 +63,16 @@ class QrUtil
     public static function encodeOldPayloadFromJson(string $jsonText): string
     {
         $json = self::normalizeJson($jsonText);
-        // Qt qCompress format: 4-byte big-endian uncompressed length + zlib compressed data
-        // This is what Amnezia Android's qUncompress expects
+        // Amnezia format: 12-byte header [version, compressed_len, uncompressed_len] + zlib compressed data
+        // Version 0x07C00100 is required for compatibility with Amnezia apps
         $compressed = gzcompress($json, 9);
         if ($compressed === false) {
             throw new RuntimeException('gzcompress failed');
         }
         $uncompressedLen = strlen($json);
-        // Qt format: 4 bytes of uncompressed length (big-endian) + zlib data
-        $header = pack('N', $uncompressedLen);
+        $compressedLen = strlen($compressed) + 4; // +4 for the uncompressed length field
+        $version = 0x07C00100; // Amnezia magic version number
+        $header = pack('N3', $version, $compressedLen, $uncompressedLen);
         return self::urlsafe_b64_encode($header . $compressed);
     }