refactor(git): model refs as domain types instead of strings

[greenbugx]

Replace the string-and-flag ref representation with a tagged union:
RefName wraps a ref's short name, Branch encodes local vs remote as
variants rather than a separate kind field, and Ref composes the two
so a badge's local/remote/tag identity lives in exactly one place.
RefKind and RefBadge are gone.

Move refs-pane flattening out of the Git layer: RepoRefs::rows and
RefRow become app::state::ref_pane_rows and RefPaneRow, since the
"Local Branches / Remote Branches / Tags" sections are presentation,
not domain. RepoRefs now carries one ordered branches vec instead of
separate local and remote lists.

No behavior change: the same refs are discovered, named, ordered and
rendered as before.