When two instances each have a native user with the same username, the
Add Friend search card for the federated one sent its request to the
local namesake instead of the intended remote user.
Root cause: `isNative = !homeUserId` in socialStore's searchUsers and
loadFriends dedup. The server backfills native users' homeUserId to
their own id so federation tier-1 lookups succeed, so `homeUserId` is
set on natives too. Only `homeInstance` distinguishes native (null)
from replicated stubs. With the wrong check, no entry was ever "native"
and the home-origin stub of the remote user was kept over the true
native record — leaving `_instanceOrigin=''`, which caused the Send
button handler to drop the domain suffix and POST to the home API,
where "nova" resolved to a completely different local user.
Also fixes loadRequests dedup to prefer the target-native record so the
search card correctly flips to "Request Pending" after sending.
Jannis Braun
aeebf79feb