fix: route collaborator saves to owner's PDS using ownerDID from request

internal/handler/handler.go

···
 	IsOwner bool
 	// IsCollaborator is true when the current user is in the collaborators list.
 	IsCollaborator bool
+	// OwnerDID is the document owner's ATProto DID. Empty when IsOwner is true.
+	// Used by collaborators to route save requests to the owner's PDS.
+	OwnerDID string
 }
 
 func (h *Handler) currentUser(r *http.Request) *model.User {
···
 	doc.RKey = rkey
 
 	editData := &DocumentEditData{Document: doc, IsOwner: isOwner}
+	if !isOwner {
+		editData.OwnerDID = ownerDID
+	}
 	if session, err := h.DB.GetATProtoSession(user.ID); err == nil && session != nil {
 		editData.AccessToken = session.AccessToken
 		userDID := session.DID
···
 
 	rkey := r.PathValue("rkey")
 	var req struct {
-		Content string `json:"content"`
-		Title   string `json:"title"`
+		Content  string `json:"content"`
+		Title    string `json:"title"`
+		OwnerDID string `json:"ownerDID"` // non-empty when saving on behalf of another user (collaborator)
 	}
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		http.Error(w, "Bad request", 400)
 		return
 	}
 
-	client, err := h.xrpcClient(user.ID)
-	if err != nil {
-		http.Error(w, "Could not connect to PDS", 500)
-		return
+	// For collaborators, save to the document owner's PDS, not the collaborator's.
+	var client *xrpc.Client
+	var repoDID string
+	if req.OwnerDID != "" {
+		ownerUser, err := h.DB.GetUserByDID(req.OwnerDID)
+		if err != nil {
+			log.Printf("APIDocumentSave: get owner by DID %s: %v", req.OwnerDID, err)
+			http.Error(w, "Document owner not found", 404)
+			return
+		}
+		client, err = h.xrpcClient(ownerUser.ID)
+		if err != nil {
+			http.Error(w, "Could not connect to owner PDS", 500)
+			return
+		}
+		repoDID = req.OwnerDID
+	} else {
+		var err error
+		client, err = h.xrpcClient(user.ID)
+		if err != nil {
+			http.Error(w, "Could not connect to PDS", 500)
+			return
+		}
+		repoDID = client.DID()
 	}
 
 	// Fetch existing record to preserve fields
-	value, _, err := client.GetRecord(client.DID(), collectionDocument, rkey)
+	value, _, err := client.GetRecord(repoDID, collectionDocument, rkey)
 	if err != nil {
 		http.Error(w, "Document not found", 404)
 		return

templates/document_edit.html

···
     const rkey = '{{.Content.RKey}}';
     const accessToken = '{{.Content.AccessToken}}';
     const isCollaborator = {{if .Content.IsCollaborator}}true{{else}}false{{end}};
+    const ownerDID = '{{.Content.OwnerDID}}'; // empty string when current user is owner
 
     const STORAGE_KEY = 'editor-mode';
     let currentMode = localStorage.getItem(STORAGE_KEY) || 'rich'; // 'rich' | 'source'
···
                 await fetch(`/api/docs/${rkey}/autosave`, {
                     method: 'PUT',
                     headers: {'Content-Type': 'application/json'},
-                    body: JSON.stringify({content, title: titleInput.value}),
+                    body: JSON.stringify({content, title: titleInput.value, ownerDID}),
                 });
                 saveStatus.textContent = 'Auto-saved';
                 saveStatus.className = 'status-saved';
···
             const resp = await fetch(`/api/docs/${rkey}/save`, {
                 method: 'POST',
                 headers: {'Content-Type': 'application/json'},
-                body: JSON.stringify({content, title: titleInput.value}),
+                body: JSON.stringify({content, title: titleInput.value, ownerDID}),
             });
             if (resp.ok) {
                 saveStatus.textContent = 'Saved!';