CmdPal: Make settings and app state immutable (#46451)

## Summary
 
 This PR refactors CmdPal settings/state to be immutable end-to-end.
 
 ### Core changes
 - Convert model types to immutable records / init-only properties:
   - `SettingsModel`
   - `AppStateModel`
   - `ProviderSettings`
   - `DockSettings`
   - `RecentCommandsManager`
- supporting settings types (fallback/hotkey/alias/top-level
hotkey/history items, etc.)
- Replace mutable collections with immutable equivalents where
appropriate:
   - `ImmutableDictionary<,>`
   - `ImmutableList<>`
 - Move mutation flow to atomic service updates:
- `ISettingsService.UpdateSettings(Func<SettingsModel, SettingsModel>)`
   - `IAppStateService.UpdateState(Func<AppStateModel, AppStateModel>)`
- Update ViewModels/managers/services to use copy-on-write (`with`)
patterns instead of in-place
mutation.
- Update serialization context + tests for immutable model graph
compatibility.
 
 ## Why
 
Issue #46437 is caused by mutable shared state being updated from
different execution paths/threads,
leading to race-prone behavior during persistence/serialization.
 
By making settings/app state immutable and using atomic swap/update
patterns, we remove in-place
mutation and eliminate this class of concurrency bug.
 
 ## Validation
 
 - Built successfully:
   - `Microsoft.CmdPal.UI.ViewModels`
   - `Microsoft.CmdPal.UI`
   - `Microsoft.CmdPal.UI.ViewModels.UnitTests`
 - Updated unit tests for immutable update patterns.
 
 Fixes #46437

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Settings/DockSettings.cs

@@ -2,6 +2,7 @@
 // The Microsoft Corporation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections.Immutable;
 using System.Text.Json.Serialization;
 using Windows.UI;
 
@@ -13,103 +14,93 @@ namespace Microsoft.CmdPal.UI.ViewModels.Settings;
 /// Settings for the Dock. These are settings for _the whole dock_. Band-specific
 /// settings are in <see cref="DockBandSettings"/>.
 /// </summary>
-public class DockSettings
+public record DockSettings
 {
-    public DockSide Side { get; set; } = DockSide.Top;
+    public DockSide Side { get; init; } = DockSide.Top;
 
-    public DockSize DockSize { get; set; } = DockSize.Small;
+    public DockSize DockSize { get; init; } = DockSize.Small;
 
-    public DockSize DockIconsSize { get; set; } = DockSize.Small;
+    public DockSize DockIconsSize { get; init; } = DockSize.Small;
 
     // <Theme settings>
-    public DockBackdrop Backdrop { get; set; } = DockBackdrop.Acrylic;
+    public DockBackdrop Backdrop { get; init; } = DockBackdrop.Acrylic;
 
-    public UserTheme Theme { get; set; } = UserTheme.Default;
+    public UserTheme Theme { get; init; } = UserTheme.Default;
 
-    public ColorizationMode ColorizationMode { get; set; }
+    public ColorizationMode ColorizationMode { get; init; }
 
-    public Color CustomThemeColor { get; set; } = new() { A = 0, R = 255, G = 255, B = 255 }; // Transparent — avoids WinUI3 COM dependency on Colors.Transparent and COM in class init
+    public Color CustomThemeColor { get; init; } = new() { A = 0, R = 255, G = 255, B = 255 }; // Transparent — avoids WinUI3 COM dependency on Colors.Transparent and COM in class init
 
-    public int CustomThemeColorIntensity { get; set; } = 100;
+    public int CustomThemeColorIntensity { get; init; } = 100;
 
-    public int BackgroundImageOpacity { get; set; } = 20;
+    public int BackgroundImageOpacity { get; init; } = 20;
 
-    public int BackgroundImageBlurAmount { get; set; }
+    public int BackgroundImageBlurAmount { get; init; }
 
-    public int BackgroundImageBrightness { get; set; }
+    public int BackgroundImageBrightness { get; init; }
 
-    public BackgroundImageFit BackgroundImageFit { get; set; }
+    public BackgroundImageFit BackgroundImageFit { get; init; }
 
-    public string? BackgroundImagePath { get; set; }
+    public string? BackgroundImagePath { get; init; }
 
     // </Theme settings>
-    // public List<string> PinnedCommands { get; set; } = [];
-    public List<DockBandSettings> StartBands { get; set; } = [];
-
-    public List<DockBandSettings> CenterBands { get; set; } = [];
-
-    public List<DockBandSettings> EndBands { get; set; } = [];
-
-    public bool ShowLabels { get; set; } = true;
-
-    [JsonIgnore]
-    public IEnumerable<(string ProviderId, string CommandId)> AllPinnedCommands =>
-        StartBands.Select(b => (b.ProviderId, b.CommandId))
-        .Concat(CenterBands.Select(b => (b.ProviderId, b.CommandId)))
-        .Concat(EndBands.Select(b => (b.ProviderId, b.CommandId)));
-
-    public DockSettings()
-    {
-        // Initialize with default values
-        // PinnedCommands = [
-        //     "com.microsoft.cmdpal.winget"
-        // ];
-        StartBands.Add(new DockBandSettings
+    public ImmutableList<DockBandSettings> StartBands { get; init; } = ImmutableList.Create(
+        new DockBandSettings
         {
             ProviderId = "com.microsoft.cmdpal.builtin.core",
             CommandId = "com.microsoft.cmdpal.home",
-        });
-        StartBands.Add(new DockBandSettings
+        },
+        new DockBandSettings
         {
             ProviderId = "WinGet",
             CommandId = "com.microsoft.cmdpal.winget",
             ShowLabels = false,
         });
 
-        EndBands.Add(new DockBandSettings
+    public ImmutableList<DockBandSettings> CenterBands { get; init; } = ImmutableList<DockBandSettings>.Empty;
+
+    public ImmutableList<DockBandSettings> EndBands { get; init; } = ImmutableList.Create(
+        new DockBandSettings
         {
             ProviderId = "PerformanceMonitor",
             CommandId = "com.microsoft.cmdpal.performanceWidget",
-        });
-        EndBands.Add(new DockBandSettings
+        },
+        new DockBandSettings
         {
             ProviderId = "com.microsoft.cmdpal.builtin.datetime",
             CommandId = "com.microsoft.cmdpal.timedate.dockBand",
         });
-    }
+
+    public bool ShowLabels { get; init; } = true;
+
+    [JsonIgnore]
+    public IEnumerable<(string ProviderId, string CommandId)> AllPinnedCommands =>
+        StartBands.Select(b => (b.ProviderId, b.CommandId))
+        .Concat(CenterBands.Select(b => (b.ProviderId, b.CommandId)))
+        .Concat(EndBands.Select(b => (b.ProviderId, b.CommandId)));
 }
 
 /// <summary>
 /// Settings for a specific dock band. These are per-band settings stored
 /// within the overall <see cref="DockSettings"/>.
 /// </summary>
-public class DockBandSettings
+public record DockBandSettings
 {
-    public required string ProviderId { get; set; }
+    public required string ProviderId { get; init; }
 
-    public required string CommandId { get; set; }
+    public required string CommandId { get; init; }
 
     /// <summary>
     /// Gets or sets whether titles are shown for items in this band.
     /// If null, falls back to dock-wide ShowLabels setting.
     /// </summary>
-    public bool? ShowTitles { get; set; }
+    public bool? ShowTitles { get; init; }
 
     /// <summary>
     /// Gets or sets whether subtitles are shown for items in this band.
     /// If null, falls back to dock-wide ShowLabels setting.
     /// </summary>
-    public bool? ShowSubtitles { get; set; }
+    public bool? ShowSubtitles { get; init; }
 
     /// <summary>
     /// Gets or sets a value for backward compatibility. Maps to ShowTitles.
@@ -118,7 +109,7 @@ public class DockBandSettings
     public bool? ShowLabels
     {
         get => ShowTitles;
-        set => ShowTitles = value;
+        init => ShowTitles = value;
     }
 
     /// <summary>
@@ -134,17 +125,6 @@ public class DockBandSettings
     /// dock-wide setting (passed as <paramref name="defaultValue"/>).
     /// </summary>
     public bool ResolveShowSubtitles(bool defaultValue) => ShowSubtitles ?? defaultValue;
-
-    public DockBandSettings Clone()
-    {
-        return new()
-        {
-            ProviderId = this.ProviderId,
-            CommandId = this.CommandId,
-            ShowTitles = this.ShowTitles,
-            ShowSubtitles = this.ShowSubtitles,
-        };
-    }
 }
 
 public enum DockSide