fix(editor): stop the query editor stealing focus on tab open; add VoiceOver labels (#1490)

[datlechin]

Part of #1490 (keyboard, focus, accessibility). Query editor surface.

Focus

• The editor's deferred auto-focus on tab open now only fires when the window has no other first responder (firstResponder == nil or the window itself), so opening a query tab while the sidebar or another control is focused no longer yanks focus into the editor. Builds on the isDestroyed guard from fix(datagrid): move focus to grid on explicit table open (#1490) #1497.

VoiceOver

• The SourceEditor gets .accessibilityLabel("SQL query editor").
• The icon-only Clear and Format toolbar buttons get accessibility labels (Execute and Explain already carry text labels).

Native APIs only. Several other gaps from the audit are already merged (filter-apply focus return in #1492) or are larger follow-ups (Tab-to-exit the editor via the key view loop, the first-responder polling refactor, the split key-view-loop wiring).

Lint clean, style gate clean. Focus/VoiceOver verified manually.