fix: avoid panic when creating a feed without any category

Deleting the last category leaves the user with no category at all.
Creating a feed with category_id omitted or set to zero then
dereferenced the nil result of FirstCategory and panicked. The same
pattern existed in the OPML import and Google Reader subscribe paths.

FirstCategory now returns ErrNoCategory instead of (nil, nil) when
the user has no category, and all three call sites translate this
error into a bad request response instead of panicking.