fix(test): give Ui.Tests task seeds a unique Number

The task-numbers slice added a UNIQUE index on tasks.number. Ui.Tests seed
TaskEntity rows directly instead of going through TaskRepository, so every row
kept the default Number = 0 and 31 tests failed with
'UNIQUE constraint failed: tasks.number'.

tests/ClaudeDo.Ui.Tests/TestTaskNumbers.cs

@@ -0,0 +1,13 @@
+namespace ClaudeDo.Ui.Tests;
+
+/// <summary>
+/// Tests seed <see cref="ClaudeDo.Data.Models.TaskEntity"/> rows directly instead of going through
+/// TaskRepository, so nothing allocates their Number — and every row would default to 0, colliding
+/// on the unique index. Hand out a process-wide unique number instead.
+/// </summary>
+internal static class TestTaskNumbers
+{
+    private static int _next;
+
+    public static int Next() => Interlocked.Increment(ref _next);
+}