Manage Databases: hide 'Select' toolbar button when no databases are imported

[jschick04]

Summary

In the Manage Databases tab of the Database Tools modal, the Select toolbar button is rendered even when no databases are imported. With nothing to select, the button is non-functional and adds visual noise alongside the empty-state message.

Repro

1. Open Database Tools → Manage Databases with no databases imported.
2. Observe both Select and Import database… are rendered above the empty-state message "Import a provider database (.db file) using the Import database… button above to resolve events from other providers."

Expected

• Import database… is always visible (only path to add content).
• Select is hidden until DatabaseService.Entries.Count > 0.

Source

• src/EventLogExpert.UI/DatabaseTools/Tabs/ManageDatabasesTab.razor — wrap the Select button (lines 15–23) in @if (DatabaseService.Entries.Count > 0).
• Make sure the surrounding manage-databases-import-row flex layout still right-aligns Import when Select is hidden.
• Add coverage to tests/Unit/EventLogExpert.UI.Tests/DatabaseTools/Tabs/ManageDatabasesEmptyStateTests.cs (or the ManageDatabasesTab tests) asserting the Select button is absent in the empty state.