Extract the Git panel into a dedicated `workspace-git-panel` Lit component
(mirroring the Files panel) and add a segmented List/Tree toggle next to
Refresh.
Tree view builds an in-memory, collapsible directory tree from the changed
files, starts fully collapsed, and offers a single expand-all/collapse-all
button (visible only in tree view). List view keeps the existing flat,
full-path rows. The selected view mode persists in localStorage under
`pi-web.gitFileView`; per-directory expand state is intentionally ephemeral.
- gitFileViewPreference.ts: localStorage-backed view preference (+ test)
- gitFileTree.ts: pure flat-paths -> nested-tree builder (+ test)
- WorkspaceGitPanel.ts: the panel component with toggle + tree state
