feat(m1): storage layer

- Shared SQLite plumbing: PRAGMAs (WAL/NORMAL/foreign_keys) and
  user_version migrations with typed SchemaVersionException
- Workspace folder: .ykanban initialization, empty schema with
  DDL-level constraints, preset column seeding (ResX-driven),
  exclusive lock file with holder diagnostics
- AppData folder: app.db workspace registry (injectable clock),
  app.json config store with kebab-case JSON names via explicit
  [JsonProperty]/[EnumMember], nested sections, corrupt-file backup
- ResX infrastructure under Assets/Locales (en + zh-Hans) with
  MSBuild strongly-typed public Resources class
- 54 unit tests: DDL constraints, migrations, PRAGMAs, lock
  exclusivity, config degradation, culture-aware presets

YKanBan.Tests/TestUtilities/SqliteTestHelper.cs

@@ -0,0 +1,67 @@
+using Microsoft.Data.Sqlite;
+using YKanBan.Storage;
+using YKanBan.Storage.Workspace;
+
+namespace YKanBan.Tests.TestUtilities;
+
+/// <summary>Shared helpers for storage tests.</summary>
+public static class SqliteTestHelper
+{
+    /// <summary>Executes a non-query SQL statement and returns the affected row count.</summary>
+    public static int Exec(SqliteConnection connection, string sql, params (string Name, object? Value)[] parameters)
+    {
+        using var command = connection.CreateCommand();
+        command.CommandText = sql;
+        foreach ((string name, object? value) in parameters)
+        {
+            command.Parameters.AddWithValue(name, value);
+        }
+        return command.ExecuteNonQuery();
+    }
+
+    /// <summary>Returns the scalar result of a SQL query as a string.</summary>
+    public static string ScalarString(SqliteConnection connection, string sql)
+    {
+        using var command = connection.CreateCommand();
+        command.CommandText = sql;
+        return (string)command.ExecuteScalar()!;
+    }
+
+    /// <summary>Returns the scalar result of a SQL query as a long.</summary>
+    public static long ScalarLong(SqliteConnection connection, string sql)
+    {
+        using var command = connection.CreateCommand();
+        command.CommandText = sql;
+        return (long)command.ExecuteScalar()!;
+    }
+
+    /// <summary>Returns true when a table with the given name exists.</summary>
+    public static bool TableExists(SqliteConnection connection, string tableName)
+    {
+        using var command = connection.CreateCommand();
+        command.CommandText = "SELECT COUNT(*) FROM sqlite_master WHERE type='table' AND name=$name;";
+        command.Parameters.AddWithValue("$name", tableName);
+        return (long)command.ExecuteScalar()! == 1;
+    }
+
+    /// <summary>Returns the titles of all columns, ordered by id.</summary>
+    public static List<string> GetColumnTitles(SqliteConnection connection)
+    {
+        using var command = connection.CreateCommand();
+        command.CommandText = "SELECT title FROM columns ORDER BY id ASC;";
+        using var reader = command.ExecuteReader();
+
+        var titles = new List<string>();
+        while (reader.Read())
+        {
+            titles.Add(reader.GetString(0));
+        }
+        return titles;
+    }
+
+    /// <summary>Opens a workspace database with the real migration list.</summary>
+    public static SqliteConnection OpenWorkspace(string folderPath)
+    {
+        return SqliteDatabase.Open(WorkspacePaths.Database(folderPath), WorkspaceSchema.Migrations);
+    }
+}