Fix SB2.1 detection offsets

The 'sgtl' magic sits at offset 52 in real SB2.1 files (with 'STMP' at
offset 20), not at offset 28. Real .sb2 release files were therefore not
recognized as SB2, and verify_firmware rejected them on locked devices
as if they were raw .bin images.

Verified against app-secure-2.2362.0.sb2 and the official
solo2-firmware-2.964.0.sb2 from GitHub releases; confirmed by a
successful flash on a real Secure device.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: manuel-domke

src/solo_gui/workers/firmware_worker.py

@@ -26,8 +26,8 @@
 
 
 def _is_sb2_file(data: bytes) -> bool:
-    """Detect SB2.1 by the 'sgtl' magic at bytes 28-32."""
-    return len(data) >= 96 and data[28:32] == b"sgtl"
+    """Detect SB2.1 by the header magics: 'STMP' at offset 20, 'sgtl' at offset 52."""
+    return len(data) >= 96 and data[20:24] == b"STMP" and data[52:56] == b"sgtl"
 
 
 @dataclass