fix: prevent duplicate DMs at creation time, clean up corrupted read states

Replace the unreliable client-side DM dedup loop in populateFromReady with
a creation-time guard (findExistingDmForUser) that checks all instances
before opening a new DM. Guards added to FriendsPage, NewDmModal, and
UserProfilePopout.

Also fixes: corrupted read_states from temp_ optimistic message IDs (server
migration + client-side validation), federation-aware closeDm/addDmMember
API routing, isSelf-based DM member filtering in sidebar/header, and WS
event error isolation.

packages/web/src/components/modals/NewDmModal.tsx

@@ -59,6 +59,13 @@ export function NewDmModal() {
   const handleSelectUser = async (user: User) => {
     setError('');
     try {
+      const existing = useServerStore.getState().findExistingDmForUser(user);
+      if (existing) {
+        closeModal();
+        useUIStore.getState().setShowDms(true);
+        navigate(`/channels/@me/${existing.dm.id}`);
+        return;
+      }
       const channel = await api.dm.create({ userId: user.id });
       addDmChannel(channel);
       closeModal();