feat: notes to self via self-DM

Backend:
- Removed self-DM restriction in conversation Create
- Added duplicate check for self-DMs (returns existing one)

Frontend:
- ConversationList: [📝] button creates/opens notes to self
- Self-DMs show as 'Notes' with 📝 icon in sidebar
- DMChat header shows 'Notes' for self-DMs

web/src/components/DMChat.tsx

@@ -75,13 +75,16 @@ export function DMChat() {
     }
   };
 
-  const title = conversation
-    ? conversation.type === "group_dm"
-      ? conversation.name || conversation.members.map((m) => m.username).join(", ")
-      : conversation.members.find((m) => m.id !== currentUser?.id)?.username || "DM"
-    : id
-      ? "[LOADING...]"
-      : "[NO CONVERSATION]";
+  const selfDM = conversation && conversation.members.length === 1 && conversation.members[0].id === currentUser?.id;
+  const title = selfDM
+    ? "Notes"
+    : conversation
+      ? conversation.type === "group_dm"
+        ? conversation.name || conversation.members.map((m) => m.username).join(", ")
+        : conversation.members.find((m) => m.id !== currentUser?.id)?.username || "DM"
+      : id
+        ? "[LOADING...]"
+        : "[NO CONVERSATION]";
 
   return (
     <div className="flex flex-col h-full bg-gb-bg">