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

internal/dm/handlers.go

@@ -82,10 +82,7 @@ func (h *Handler) Create(w http.ResponseWriter, r *http.Request) {
 			memberSet[uid] = struct{}{}
 		}
 	}
-	if len(memberSet) == 1 {
-		http.Error(w, `{"error":"cannot create conversation with yourself"}`, http.StatusBadRequest)
-		return
-	}
+	// ponytail: allow self-DM (notes to self) — len(memberSet)==1 is fine
 
 	memberIDs := make([]string, 0, len(memberSet))
 	for uid := range memberSet {
@@ -94,7 +91,24 @@ func (h *Handler) Create(w http.ResponseWriter, r *http.Request) {
 	sort.Strings(memberIDs)
 
 	// For a 1:1 DM, check if it already exists.
-	if len(memberIDs) == 2 {
+	if len(memberIDs) == 1 {
+		var existingID string
+		err := h.db.QueryRowContext(r.Context(), `
+			SELECT c.id FROM conversations c
+			WHERE c.type = 'dm'
+			  AND (SELECT COUNT(*) FROM conversation_members cm WHERE cm.conversation_id = c.id) = 1
+			  AND EXISTS (SELECT 1 FROM conversation_members cm2 WHERE cm2.conversation_id = c.id AND cm2.user_id = $1)
+			LIMIT 1
+		`, memberIDs[0]).Scan(&existingID)
+		if err == nil {
+			h.getByID(w, r, existingID)
+			return
+		}
+		if !errors.Is(err, sql.ErrNoRows) {
+			http.Error(w, `{"error":"server error"}`, http.StatusInternalServerError)
+			return
+		}
+	} else if len(memberIDs) == 2 {
 		var existingID string
 		err := h.db.QueryRowContext(r.Context(), `
 			SELECT c.id FROM conversations c