frontend: Ensure cookie ID is always 16 hexadecimal characters

At a glance, the GenId function looks like it can only return a
16-character hexadecimal string with all characters being [0-9A-F].
However, it seems that it can rarely return a 16-character string that
has one or two space characters at the beginning due to the value of id,
from which the final string is derived, being too low (lower than
1152921504606846976 or 0x1000000000000000) and the printf format
specifier having a width of 16. This results in a string of less than 16
characters that is padded with blank spaces. The end result is a cookie
directory that has leading spaces in its name, which can cause various
issues, such as breaking syncing on OneDrive.

If we set the format specifier to pad with zeroes instead of spaces, the
resulting hexadecimal value is always 16 characters long without spaces.

frontend/widgets/OBSBasic_Browser.cpp

@@ -169,7 +169,7 @@ static std::string GenId()
 	uint64_t id = dist(e2);
 
 	char id_str[20];
-	snprintf(id_str, sizeof(id_str), "%16llX", (unsigned long long)id);
+	snprintf(id_str, sizeof(id_str), "%016llX", (unsigned long long)id);
 	return std::string(id_str);
 }