feat: flag vendored clones and make the repo filter navigable

Two Repos-tab quality-of-life changes:

- External/vendored clones (repos with zero of your own commits — e.g. an
  upstream cloned for reference and never pushed to) are measured on
  refresh, cached in stats_cache (RepoStatsEntry.authored), shown with a
  dim ⧉ marker prefixed to the name, and hidden by the persistent
  hide_external_clones config toggle (key `x`, the `S` menu, or
  `vulp list --external|--no-external`). A fork you do commit to stays
  yours, so only true reference clones are flagged.

- Inside the `/` filter box, ↑↓ now move through the live-filtered
  results and Enter opens the highlighted repo/project directly, instead
  of the filter only "locking" on Enter with no way to scroll or select.

src/hub/modals_confirm_sync.rs

@@ -26,6 +26,10 @@ pub fn try_confirm(state: &mut HubState, modal: Modal) -> Option<Modal> {
                 7 => state.filter_ahead    = !state.filter_ahead,
                 8 => state.filter_archived = !state.filter_archived,
                 9 => {
+                    state.cfg.hide_external_clones = !state.cfg.hide_external_clones;
+                    let _ = crate::config::save_config(&state.cfg);
+                }
+                10 => {
                     state.sort_mode      = SortMode::Default;
                     state.filter_dirty   = false;
                     state.filter_behind  = false;