feat(ui): drag-and-drop to open .nest files in Database mode

Open browses server records in Database mode, with no way to load a local
file. Dragging .nest files onto the window (or an open plate) now opens
them from disk in either storage mode; PlateView forwards file drops via a
new FilesDropped event instead of swallowing them, gated on having a
listener so other PlateView hosts are unaffected. A dropped document is
unbound, so Save in Database mode creates a new record on first save via
the existing storage-mode logic.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

OpenNest/Controls/PlateView.cs

@@ -78,6 +78,12 @@ namespace OpenNest.Controls
         public event EventHandler<ItemRemovedEventArgs<Part>> PartRemoved;
         public event EventHandler StatusChanged;
 
+        /// <summary>
+        /// Raised when files are dropped on the plate and something is listening (see
+        /// <see cref="OnDragEnter"/>); callers decide which paths to act on.
+        /// </summary>
+        public event EventHandler<string[]> FilesDropped;
+
         public event EventHandler SelectionChanged
         {
             add => selection.SelectionChanged += value;
@@ -305,10 +311,18 @@ namespace OpenNest.Controls
         {
             if (drgevent.Data.GetData(typeof(Drawing)) != null)
                 drgevent.Effect = DragDropEffects.Copy;
+            else if (FilesDropped != null && drgevent.Data.GetDataPresent(DataFormats.FileDrop))
+                drgevent.Effect = DragDropEffects.Copy;
         }
 
         protected override void OnDragDrop(DragEventArgs drgevent)
         {
+            if (drgevent.Data.GetDataPresent(DataFormats.FileDrop))
+            {
+                FilesDropped?.Invoke(this, (string[])drgevent.Data.GetData(DataFormats.FileDrop));
+                return;
+            }
+
             var dwg = drgevent.Data.GetData(typeof(Drawing)) as Drawing;
 
             if (dwg == null)

OpenNest/Forms/MainForm.cs

@@ -69,6 +69,7 @@ namespace OpenNest.Forms
             clickUpdateLocation = true;
 
             this.SetBevel(false);
+            AllowDrop = true;
 
             LoadPosts();
             EnableCheck();
@@ -173,6 +174,7 @@ namespace OpenNest.Forms
             if (remoteId != Guid.Empty)
                 editForm.Document.BindRemote(remoteId, serverUrl);
             editForm.MdiParent = this;
+            editForm.PlateView.FilesDropped += (sender, paths) => OpenDroppedNestFiles(paths);
             editForm.PlateChanged += (sender, e) =>
             {
                 NavigationEnableCheck();
@@ -552,6 +554,43 @@ namespace OpenNest.Forms
             Settings.Default.Save();
         }
 
+        protected override void OnDragEnter(DragEventArgs e)
+        {
+            base.OnDragEnter(e);
+            e.Effect = HasDroppedNestFile(e.Data) ? DragDropEffects.Copy : DragDropEffects.None;
+        }
+
+        protected override void OnDragDrop(DragEventArgs e)
+        {
+            base.OnDragDrop(e);
+            if (e.Data.GetData(DataFormats.FileDrop) is string[] paths)
+                OpenDroppedNestFiles(paths);
+        }
+
+        private static bool HasDroppedNestFile(IDataObject data) =>
+            data.GetData(DataFormats.FileDrop) is string[] paths
+            && paths.Any(path => string.Equals(
+                Path.GetExtension(path), NestFormat.FileExtension, StringComparison.OrdinalIgnoreCase));
+
+        /// <summary>
+        /// Opens dropped .nest files locally regardless of storage mode. Loaded documents are
+        /// not bound to a server record, so Save/Save As follow the normal storage-mode rules
+        /// (File mode writes to disk; Database mode creates a new record on first save).
+        /// </summary>
+        private void OpenDroppedNestFiles(IEnumerable<string> paths)
+        {
+            foreach (var path in paths)
+            {
+                if (!string.Equals(Path.GetExtension(path), NestFormat.FileExtension, StringComparison.OrdinalIgnoreCase))
+                    continue;
+
+                var reader = new NestReader(path);
+                var nest = reader.Read();
+                LoadNest(nest);
+                ShowNestWarnings(reader);
+            }
+        }
+
         protected override void OnClosing(System.ComponentModel.CancelEventArgs e)
         {
             base.OnClosing(e);

docs/nest-storage.md

@@ -18,6 +18,13 @@ The desktop app can save nests two ways:
   modes) for producing a local file to share or back up. Export does not change
   the document's file save path or database record association.
 
+Dragging one or more `.nest` files onto the main window (or onto an open
+plate) opens them from disk directly, in either mode — this is the only way
+to open a local file while in Database mode, since Open there browses server
+records instead of showing a file dialog. A document opened this way is not
+bound to a server record, so a subsequent Save in Database mode creates a new
+record on first save, same as any other unbound document.
+
 The mode and server address are stored per-PC at `%APPDATA%\OpenNest\storage.json`
 (`OpenNest.Data.NestStorageSettings`), defaulting to File mode so existing
 installs are unaffected until an operator opts in via **File > Storage Mode...**.