Fix iTunesCDB header: always generate proper mhfd header

- Remove broken header-preservation logic (was reusing all-zero header)
- Always write header with correct total_size at offset 8
- Add version field 0x00020002 at offset 12
- Delete old iTunesDB (iTunesDB.old) to avoid firmware confusion

src/ipod_nano7_db.py

@@ -1103,47 +1103,19 @@ class Nano7Database:
         # Compute new SHA1 hash
         new_hash = hashlib.sha1(lib_data).hexdigest()
         
-        # Read existing iTunesCDB to extract the header (first bytes before zlib)
-        if not os.path.exists(self.cdb_path):
-            header = b"\x00" * 244
-            header = b"mhfd" + struct.pack("<I", 244) + header[8:]
         compressed = zlib.compress(lib_data)
-            with open(self.cdb_path, "wb") as f:
-                f.write(header + compressed)
-            logger.info(f"Created iTunesCDB: compressed={len(compressed)} bytes")
-        else:
-            with open(self.cdb_path, "rb") as f:
-                cdb_data = f.read()
+        total_size = 244 + len(compressed)
 
-            # Find zlib header (0x78 0x9c is most common, but also check 0x78 0xda and 0x78 0x01)
-            zlib_offset = -1
-            for pattern in [b"\x78\x9c", b"\x78\xda", b"\x78\x01", b"\x78\x5e"]:
-                zlib_offset = cdb_data.find(pattern)
-                if zlib_offset >= 0:
-                    break
-            if zlib_offset == -1:
-                logger.error("Could not find zlib header in iTunesCDB")
-                return
-
-            header = cdb_data[:zlib_offset]
-
-            # Compress Library.itdb
-            compressed = zlib.compress(lib_data)
+        # Build proper mhfd header
+        header = bytearray(244)
+        header[0:4] = b"mhfd"
+        struct.pack_into("<I", header, 4, 244)
+        struct.pack_into("<I", header, 8, total_size)
+        struct.pack_into("<I", header, 12, 0x00020002)
 
         # Write new iTunesCDB
         with open(self.cdb_path, "wb") as f:
-                f.write(header + compressed)
-        
-        # Update hash in iTunesCDB.ext
-        if not os.path.exists(self.cdb_ext_path):
-            ext_content = f"[iTunesCDB]\nitunesdb_hash={new_hash}\n"
-        else:
-            with open(self.cdb_ext_path, "r") as f:
-                ext_content = f.read()
-
-        new_ext_content = re.sub(r"itunesdb_hash=\w+", f"itunesdb_hash={new_hash}", ext_content)
-        with open(self.cdb_ext_path, "w") as f:
-            f.write(new_ext_content)
+            f.write(bytes(header) + compressed)
 
         logger.info(f"Synced iTunesCDB: hash={new_hash}, compressed={len(compressed)} bytes")