refactor(config): consolidate model aliases into ModelRegistry

Replaces three scattered model lists (ListSettingsModalViewModel,
DetailsIslandViewModel, GeneralSettingsTabViewModel) and the hardcoded
planning model with a single source. Planning launcher now uses the
opus alias instead of pinning claude-opus-4-7.

src/ClaudeDo.Data/Models/ModelRegistry.cs

@@ -0,0 +1,12 @@
+namespace ClaudeDo.Data.Models;
+
+public static class ModelRegistry
+{
+    public static readonly IReadOnlyList<string> Aliases = new[] { "sonnet", "opus", "haiku" };
+
+    public const string DefaultAlias = "sonnet";
+    public const string PlanningAlias = "opus";
+
+    public const string ListDefaultSentinel = "(default)";
+    public const string TaskInheritSentinel = "(inherit)";
+}