Fix: log AddCollaboration error in early-return path; use non-nil ownDocs slice

internal/handler/handler.go

···
 		h.render(w, "documents.html", PageData{Title: "Documents", User: user, UserHandle: userHandle, Content: empty})
 		return
 	}
-	var ownDocs []*model.Document
+	ownDocs := make([]*model.Document, 0, len(records))
 	for _, rec := range records {
 		doc := &model.Document{}
 		if err := json.Unmarshal(rec.Value, doc); err != nil {
···
 	// Already a collaborator — redirect to the owner-scoped URL.
 	for _, c := range doc.Collaborators {
 		if c == collabSession.DID {
-			h.DB.AddCollaboration(collabSession.DID, invite.CreatedBy, rKey) // ensure row exists
+			if err := h.DB.AddCollaboration(collabSession.DID, invite.CreatedBy, rKey); err != nil {
+				log.Printf("AcceptInvite: record collaboration (existing): %v", err)
+			}
 			http.Redirect(w, r, "/docs/"+invite.CreatedBy+"/"+rKey, http.StatusSeeOther)
 			return
 		}