feat: unified editor layout - remove OTB tab, add OTB panel + sprite strip

- Remove pill toggle (OTB Items / Client Items tabs)
- Remove separate OTB VIEW entirely
- Unified 3-column layout: client list | editor+OTB detail | OTB items list
- Move sprites from right column to horizontal bottom strip
- Add OTB panel with search, pagination, bidirectional linking
- Double-click client item opens editor + linked OTB detail
- Double-click OTB item opens OTB detail panel
- Animation timer starts on client data load

Author: BrewertonSantos

src/App/AppSettings.cs

@@ -0,0 +1,42 @@
+using System.Text.Json;
+
+namespace POriginsItemEditor.App;
+
+public class AppSettings
+{
+    public string? LastOtbPath { get; set; }
+    public string? LastClientFolderPath { get; set; }
+
+    private static string SettingsFilePath =>
+        Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+            "POriginsItemEditor",
+            "settings.json");
+
+    public static AppSettings Load()
+    {
+        try
+        {
+            var path = SettingsFilePath;
+            if (File.Exists(path))
+            {
+                var json = File.ReadAllText(path);
+                return JsonSerializer.Deserialize<AppSettings>(json) ?? new AppSettings();
+            }
+        }
+        catch { }
+        return new AppSettings();
+    }
+
+    public void Save()
+    {
+        try
+        {
+            var path = SettingsFilePath;
+            Directory.CreateDirectory(Path.GetDirectoryName(path)!);
+            var json = JsonSerializer.Serialize(this, new JsonSerializerOptions { WriteIndented = true });
+            File.WriteAllText(path, json);
+        }
+        catch { }
+    }
+}

src/App/MainWindow.axaml

@@ -1,6 +1,7 @@
 using Avalonia.Controls;
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
+using Avalonia.Interactivity;
 using POriginsItemEditor.App.ViewModels;
 
 namespace POriginsItemEditor.App;
@@ -10,6 +11,11 @@ public partial class MainWindow : Window
     public MainWindow()
     {
         InitializeComponent();
+        Loaded += async (_, _) =>
+        {
+            if (DataContext is MainWindowViewModel vm)
+                await vm.TryLoadLastSessionAsync();
+        };
     }
 
     private void OnMinimapSwatchClicked(object? sender, PointerPressedEventArgs e)
@@ -23,7 +29,6 @@ private void OnMinimapColorPicked(object? sender, PointerPressedEventArgs e)
         if (sender is not Border border || DataContext is not MainWindowViewModel vm || vm.SelectedItem is not { } item)
             return;
 
-        // Tag may be boxed as ushort or int depending on XAML binding
         ushort index = border.Tag switch
         {
             ushort u => u,
@@ -34,7 +39,6 @@ private void OnMinimapColorPicked(object? sender, PointerPressedEventArgs e)
 
         item.MinimapColor = index;
 
-        // Walk up to find the control with the attached flyout and close it
         Control? current = border;
         while (current != null)
         {
@@ -47,4 +51,26 @@ private void OnMinimapColorPicked(object? sender, PointerPressedEventArgs e)
             current = current.Parent as Control;
         }
     }
+
+    private void OnCompositionSpriteDoubleTapped(object? sender, TappedEventArgs e)
+    {
+        if (sender is Border border
+            && border.DataContext is SpriteViewModel svm
+            && DataContext is MainWindowViewModel vm)
+        {
+            vm.NavigateRightSpriteToIdCommand.Execute(svm.SpriteId);
+        }
+    }
+
+    private void OnClientItemDoubleTapped(object? sender, TappedEventArgs e)
+    {
+        if (DataContext is MainWindowViewModel vm)
+            vm.OpenClientItemEditor();
+    }
+
+    private void OnOtbItemDoubleTapped(object? sender, TappedEventArgs e)
+    {
+        if (DataContext is MainWindowViewModel vm)
+            vm.OpenOtbItemEditor();
+    }
 }

src/App/MainWindow.axaml.cs

@@ -0,0 +1,49 @@
+using Avalonia.Media.Imaging;
+using CommunityToolkit.Mvvm.ComponentModel;
+using POriginsItemEditor.OTB;
+
+namespace POriginsItemEditor.App.ViewModels;
+
+/// <summary>
+/// VM for a client item (DAT perspective) in the client items list.
+/// </summary>
+public partial class ClientItemViewModel : ObservableObject
+{
+    public DatThingType ThingType { get; }
+
+    public ClientItemViewModel(DatThingType thingType)
+    {
+        ThingType = thingType;
+    }
+
+    public ushort Id => ThingType.Id;
+    public ThingCategory Category => ThingType.Category;
+    public string CategoryName => Category.ToString();
+    public uint FirstSpriteId => ThingType.FirstSpriteId;
+
+    [ObservableProperty] private WriteableBitmap? _sprite;
+
+    public int TotalSprites => ThingType.TotalSpriteCount;
+
+    /// <summary>Current animation frame (cycled by timer in MainWindowViewModel).</summary>
+    public int AnimFrame { get; set; }
+
+    public int AnimPhases
+    {
+        get
+        {
+            if (ThingType.FrameGroups.Length == 0) return 0;
+            return ThingType.FrameGroups[0].Frames;
+        }
+    }
+
+    // ── Frame group info ──
+    public byte Width => ThingType.FrameGroups.Length > 0 ? ThingType.FrameGroups[0].Width : (byte)1;
+    public byte Height => ThingType.FrameGroups.Length > 0 ? ThingType.FrameGroups[0].Height : (byte)1;
+    public byte ExactSize => ThingType.FrameGroups.Length > 0 ? ThingType.FrameGroups[0].ExactSize : (byte)32;
+    public byte Layers => ThingType.FrameGroups.Length > 0 ? ThingType.FrameGroups[0].Layers : (byte)1;
+    public byte PatternX => ThingType.FrameGroups.Length > 0 ? ThingType.FrameGroups[0].PatternX : (byte)1;
+    public byte PatternY => ThingType.FrameGroups.Length > 0 ? ThingType.FrameGroups[0].PatternY : (byte)1;
+    public byte PatternZ => ThingType.FrameGroups.Length > 0 ? ThingType.FrameGroups[0].PatternZ : (byte)1;
+    public byte Frames => ThingType.FrameGroups.Length > 0 ? ThingType.FrameGroups[0].Frames : (byte)1;
+}

src/App/ViewModels/ClientItemViewModel.cs

@@ -124,6 +124,9 @@ private void DecrementClientId()
     [ObservableProperty] private bool _hasMismatch;
     [ObservableProperty] private uint _firstSpriteId;
 
+    /// <summary>Full DAT thing type (set during cross-reference).</summary>
+    public DatThingType? DatThingType { get; set; }
+
     public string MismatchDetail
     {
         get