Track previously seen host types and preserve them when one data source
temporarily has no entry for a hostname. This prevents the "Host" or
"Docker" type badge from disappearing and reappearing when websocket
updates cause momentary state inconsistencies.
The fix only preserves types if the corresponding source array has any
data at all, ensuring that intentional host removal (both arrays empty
for that host) still works correctly.
Related to #773
53a2c2a587