fix(storage): initialize workspaces under the lock in one transaction

Follows PLAN 4.2: create .ykanban, take the lock, then build the database.
SqliteDatabase.Create writes every migration and the preset columns in a
single transaction and deletes the file again on failure, so no empty
version-0 database is left behind. WorkspaceSession.Initialize keeps the lock
into the session instead of releasing and re-acquiring it.

A lock conflict while initializing shows the lock-conflict page.

YKanBan.Tests/Storage/Workspace/WorkspacePresetTests.cs

@@ -29,8 +29,7 @@ public class WorkspacePresetTests
             ];
 
             using var directory = new TempDirectory();
-            WorkspaceInitializer.Initialize(directory.FullPath);
-            WorkspacePreset.AddPresetColumns(directory.FullPath);
+            WorkspaceSession.Initialize(directory.FullPath).Dispose();
 
             using SqliteConnection connection = SqliteTestHelper.OpenWorkspace(directory.FullPath);
             CollectionAssert.AreEqual(expected, SqliteTestHelper.GetColumnTitles(connection));
@@ -56,8 +55,7 @@ public class WorkspacePresetTests
             ];
 
             using var directory = new TempDirectory();
-            WorkspaceInitializer.Initialize(directory.FullPath);
-            WorkspacePreset.AddPresetColumns(directory.FullPath);
+            WorkspaceSession.Initialize(directory.FullPath).Dispose();
 
             using SqliteConnection connection = SqliteTestHelper.OpenWorkspace(directory.FullPath);
             CollectionAssert.AreEqual(expected, SqliteTestHelper.GetColumnTitles(connection));
@@ -83,8 +81,7 @@ public class WorkspacePresetTests
             ];
 
             using var directory = new TempDirectory();
-            WorkspaceInitializer.Initialize(directory.FullPath);
-            WorkspacePreset.AddPresetColumns(directory.FullPath);
+            WorkspaceSession.Initialize(directory.FullPath).Dispose();
 
             // Switching the language after adding the presets must not rewrite stored data.
             Resources.Culture = new CultureInfo("zh-Hans");