imogen album create "" makes an album no name can reach #43

Symptom

AlbumCommand::Create passes the name to the API as it stands, so an empty or
whitespace-only one is accepted and stored. The album that results is reachable only by id:
find_album refuses an empty reference (#38), and no non-empty reference matches an empty
name, so imogen album show, add, remove, delete and --album can never name it
again. album update --name "" does the same to an album that had a usable name.

imogen upload --album "$ALBUM" with the variable unset is now refused, which is #38 —
this is the other half, where the unusable name is written rather than read.

Reproduction

imogen album create ""          # or --name "   "
imogen album list               # a row with an empty NAME
imogen album show ""            # refused: an empty reference cannot pick one

Expected

An album name is trimmed and cannot be empty, on create and on update --name: refuse
with a message saying so, before anything reaches the wire, the way an empty reference is
refused when reading.

The lookup side already trims the stored name when comparing (see named_album in
src/context.rs), so a padded name is at least still findable; an empty one cannot be.

Affected repos

Only imogen-cli — the server accepts what it is sent, and nothing on the wire changes.
Whether the server should also refuse an empty album name is a separate question for
imogen-server.

Context

Found by review on the #38 PR (#42), which hardens reading a reference
that cannot pick an album and leaves writing one untouched.