fix(storage): refuse to open a workspace whose database file is missing

With .ykanban present but ykanban.db gone, opening used to create an empty
database and show an empty board without any hint. It now throws
WorkspaceDatabaseMissingException and shows the open-failed page, telling the
user to restore a backup or delete .ykanban and initialize again.

YKanBan.Tests/Storage/Workspace/WorkspaceSessionTests.cs

@@ -89,4 +89,19 @@ public class WorkspaceSessionTests
         Assert.ThrowsExactly<InvalidOperationException>(() => WorkspaceSession.Initialize(directory.FullPath));
         using WorkspaceSession reopened = WorkspaceSession.Open(directory.FullPath);
     }
+
+    [TestMethod]
+    public void OpenWithMissingDatabaseThrowsAndCreatesNothing()
+    {
+        using var directory = new TempDirectory();
+        WorkspaceInitializer.Initialize(directory.FullPath);
+        string databasePath = WorkspacePaths.Database(directory.FullPath);
+        File.Delete(databasePath);
+
+        Assert.ThrowsExactly<WorkspaceDatabaseMissingException>(() => WorkspaceSession.Open(directory.FullPath));
+        Assert.IsFalse(File.Exists(databasePath));
+
+        // The lock was released again: the next attempt fails the same way, not with a lock conflict.
+        Assert.ThrowsExactly<WorkspaceDatabaseMissingException>(() => WorkspaceSession.Open(directory.FullPath));
+    }
 }

YKanBan.Tests/ViewModels/OpenFailedPageViewModelTests.cs

@@ -43,6 +43,9 @@ public class OpenFailedPageViewModelTests
             Assert.AreEqual(
                 Expected(Resources.Error_OpenFailed_SchemaTooNew, 3L, 1),
                 OpenFailedPageViewModel.BuildMessage(Folder, new SchemaVersionException(3, 1)));
+            Assert.AreEqual(
+                Expected(Resources.Error_OpenFailed_DatabaseMissing),
+                OpenFailedPageViewModel.BuildMessage(Folder, new WorkspaceDatabaseMissingException("/work/repo/.ykanban/ykanban.db")));
             Assert.AreEqual(
                 Expected(Resources.Error_OpenFailed_Database),
                 OpenFailedPageViewModel.BuildMessage(Folder, new SqliteException("file is not a database", 26)));