fix(multi-account): reset lastContactListCreatedAt on cache clear

Root cause of 'follows at 0 after 2+ switches':

DesktopLocalCache.clear() reset _followedUsers to empty but did NOT
reset lastContactListCreatedAt. On re-subscribe after account switch,
the contact list event arrived with the same timestamp, was rejected
by the 'event.createdAt <= lastContactListCreatedAt' guard, and
followedUsers stayed empty.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/cache/DesktopLocalCache.kt

@@ -612,6 +612,7 @@ class DesktopLocalCache : ICacheProvider {
         _followedUsers.value = emptySet()
         followerCounts.clear()
         followingCounts.clear()
+        lastContactListCreatedAt = 0L
     }
 }