fix: Default to 0644 when IPA entries have no Unix permissions (#13)

IPAs zipped on Windows store NTFS attributes in the ZIP external file
attributes and leave the Unix mode bits (the high 16 bits of external_fa)
unset. unzipArchiveAtURL: then derived a permission of 0 from those entries
and applied it with setAttributes:, leaving the extracted files with mode
0000. Subsequent reads during signing failed, surfacing to the user as
"You don't have permission".

Fall back to 0644 when no Unix mode bits are present so these archives
extract with readable permissions.

Fixes SideStore/SideStore#447

Author: mikey820

AltSign/Categories/NSFileManager+Zip.m

@@ -220,6 +220,11 @@ - (BOOL)unzipArchiveAtURL:(NSURL *)archiveURL toDirectory:(NSURL *)directoryURL
             } while (result > 0);
 
             short permissions = (info.external_fa >> 16) & 0x01FF;
+            if (permissions == 0) {
+                // No Unix mode bits set -- archive was likely created on Windows.
+                // Use a readable default so signing can proceed. (Fixes issue #447.)
+                permissions = 0644;
+            }
             if (![self setAttributes:@{NSFilePosixPermissions: @(permissions)} ofItemAtPath:fileURL.path error:error])
             {
                 finish();