Fix onboarding crash

This commit fixes a crash that occurred when clicking "follow all"
during onboarding.

This fix works by making `Contacts` and `PostBox` isolated into a
specific Swift Actor, and updating direct and indirect usages
accordingly.

Changelog-Fixed: Fixed a crash that occurred when clicking "follow all" during onboarding.
Closes: https://github.com/damus-io/damus/issues/3422
Co-authored-by: alltheseas <64376233+alltheseas@users.noreply.github.com>
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damus/Features/Search/Views/SearchResultsView.swift

@@ -206,7 +206,7 @@ struct SearchResultsView_Previews: PreviewProvider {
 }
  */
 
-
+@MainActor
 func search_for_string(profiles: Profiles, contacts: Contacts, search new: String) -> Search? {
     guard new.count != 0 else {
         return nil
@@ -267,6 +267,7 @@ func make_hashtagable(_ str: String) -> String {
     return String(new.filter{$0 != " "})
 }
 
+@MainActor
 func search_profiles(profiles: Profiles, contacts: Contacts, search: String) -> [Pubkey] {
     // Search by hex pubkey.
     if let pubkey = hex_decode_pubkey(search),