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>

damusTests/WalletConnectTests.swift

@@ -94,8 +94,9 @@ final class WalletConnectTests: XCTestCase {
         XCTAssertEqual(pool.all_descriptors.count, 1)
         XCTAssertEqual(pool.all_descriptors[0].variant, .nwc)
         XCTAssertEqual(pool.all_descriptors[0].url.url.absoluteString, "ws://127.0.0.1")
-        XCTAssertEqual(box.events.count, 1)
-        let ev = box.events.first!.value
+        let boxEventCount = await box.events.count
+        XCTAssertEqual(boxEventCount, 1)
+        let ev = await box.events.first!.value
         XCTAssertEqual(ev.skip_ephemeral, false)
         XCTAssertEqual(ev.remaining.count, 1)
         XCTAssertEqual(ev.remaining[0].relay.url.absoluteString, "ws://127.0.0.1")