chat: make sure we don't override existing saga values

[arthyn]

OTT, this should fix LAND-1181

[linear]

LAND-1181 old groups: version in/compatibility indicators misbehaving for chat

In old groups, when in a group for whose channels the hosts have upgraded, we correctly see a red version mismatch dot for collections, but chat channels continue displaying a green dot despite their host having upgraded. This is bad, because it makes the "we have not received the update yet, but the channel host has" case invisible & confusing to the user.

We have observed that:

• /app/chat does store a net=[%sub host=~zod load=%.y saga=[%dex ver=1]] in the channel state correctly,
• and that the frontend does receive a saga: { ahead: '1' } for the relevant channel from its /groups-ui/init/v0.json scry.

However, unclear at this time why it's not reflected in the status dot.

We have additionally observed the status dot being finnicky, sometimes staying grey until the profile of the host ship is clicked, which prompts connection re-try and turns it green, but unclear if that is related.

In any case, the primary problem here is that current mainnet isn't guaranteed to display a correct connectivity/version status indicator for chat channels, when it should just be consistent in giving a red dot for version mismatches. We may not be able to fix this without an "appetizer ota".