feat(tui): include backlinks in link picker

Link picker now shows both outgoing [[links]] and backlinks in
a unified list with section headers. Backlinks follow by entity
ID directly, outgoing links resolve by text. Navigating into a
backlink works the same as following an outgoing link — pushes
to nav stack, esc pops back.

internal/tui/commands.go

@@ -222,6 +222,16 @@ func followLink(store *db.Store, linkText string) tea.Cmd {
 	}
 }
 
+func followLinkByID(store *db.Store, entityID string) tea.Cmd {
+	return func() tea.Msg {
+		entity, err := store.Get(context.Background(), entityID)
+		if err != nil {
+			return errMsg{err}
+		}
+		return linkFollowedMsg{entity}
+	}
+}
+
 func loadRailTags(store *db.Store) tea.Cmd {
 	return func() tea.Msg {
 		tags, err := store.ListTags(context.Background(), false)