fix(repo-appearance): open icon image picker at the repo's working directory
SwiftUI's `.fileImporter()` doesn't expose an initial directory, so the
"Choose Image…" panel always opened at the user's last-used location
(usually `~/Documents` or `~/Downloads`). Most users keep their icon
asset next to the project, so they had to navigate to the repo by hand
every time.
Switch to `NSOpenPanel.begin` and seed `directoryURL` from
`store.rootURL` so Finder lands directly inside the repo. Drop the now-
redundant `isImageImporterPresented` state and `.fileImporter` modifier;
the rest of the import flow (security-scoped access isn't needed for
`NSOpenPanel`-returned URLs in this non-sandboxed app) collapses to a
single `store.send(.importUserImage(url))` on `.OK`.
The reducer's `userImageImportFailed` action stays — it's still emitted
from the in-reducer copy-failure path.