Handle DockSize enum changes (#47212)

### ⚠️ REVIEWERS TO DECIDE. MERGE THIS PR OR GO FOR: #47214⚠️

### Root cause

PR #46699 (compact mode) replaced the original `DockSize` enum
(`Small`/`Medium`/`Large`) with a new one (Default/Compact). Existing
users have `DockSize`: `Small` (or `Medium`/`Large`) in their
`settings.json`, which the source-generated EnumConverter can't parse →
the entire settings file fails to load.

### Fix

Added a custom `JsonConverter<DockSize>` in
_src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Settings/DockSettings.cs_
and applied it via `[JsonConverter]` on the enum.

The converter:

- Accepts current values (`Default`, `Compact`) — case-insensitive.
- Maps any unknown legacy string (e.g. `Small`/`Medium`/`Large`) to
`DockSize.Default`, preserving the user's existing visual experience.
- Tolerates numeric values too, falling back to `Default` if out of
range.
- Writes as a string so output stays consistent with the source-gen
`UseStringEnumConverter` setting.

Author: niels9001

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

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Immutable;
+using System.Text.Json;
 using System.Text.Json.Serialization;
 using Windows.UI;
 
@@ -135,12 +136,52 @@ public enum DockSide
     Bottom = 3,
 }
 
+[JsonConverter(typeof(DockSizeJsonConverter))]
 public enum DockSize
 {
     Default,
     Compact,
 }
 
+/// <summary>
+/// Custom converter for <see cref="DockSize"/> that preserves backward
+/// compatibility with previously-persisted values. Earlier builds shipped a
+/// <c>Small</c>/<c>Medium</c>/<c>Large</c> enum; those values are migrated to
+/// <see cref="DockSize.Default"/> so existing settings.json files continue to
+/// load instead of failing the entire deserialization.
+/// </summary>
+internal sealed class DockSizeJsonConverter : JsonConverter<DockSize>
+{
+    public override DockSize Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        if (reader.TokenType == JsonTokenType.String)
+        {
+            var value = reader.GetString();
+            if (Enum.TryParse<DockSize>(value, ignoreCase: true, out var parsed))
+            {
+                return parsed;
+            }
+
+            // Legacy values from the original Small/Medium/Large enum, or any
+            // other unknown string — fall back to Default so the user's
+            // settings file remains loadable after upgrading.
+            return DockSize.Default;
+        }
+
+        if (reader.TokenType == JsonTokenType.Number && reader.TryGetInt32(out var number))
+        {
+            return Enum.IsDefined(typeof(DockSize), number) ? (DockSize)number : DockSize.Default;
+        }
+
+        return DockSize.Default;
+    }
+
+    public override void Write(Utf8JsonWriter writer, DockSize value, JsonSerializerOptions options)
+    {
+        writer.WriteStringValue(value.ToString());
+    }
+}
+
 public enum DockBackdrop
 {
     Transparent,