fix: route Cmd+T through newWindowForTab responder chain to prevent focus flash

[datlechin]

Summary

• Bug: in a native NSWindow tab group, Cmd+T opens the new query tab and it briefly becomes key, then focus jumps back to the previous tab.
• Root cause: SwiftUI menu's Button("New Tab") called actions?.newTab() directly, bypassing AppKit's tab-aware focus management. After the menu key-equivalent dispatch returned, AppKit reasserted the previously-key window.
• Fix: route Cmd+T through the standard AppKit responder chain by overriding newWindowForTab(_:) on EditorWindow (matches the existing performClose(_:) override pattern), and dispatching from the SwiftUI menu and toolbar button via NSApp.sendAction(#selector(NSWindow.newWindowForTab(_:)), to: nil, from: nil) — the same pattern already used for selectNextTab(_:) / selectPreviousTab(_:) in this file.

Test plan

• Open a table tab, press Cmd+T → new query tab stays focused, no flash back.
• Click the toolbar + button → same behavior.
• Empty connection window (no tabs yet) → Cmd+T adds an in-window query tab.
• Multiple connection windows open → Cmd+T opens new tab in the focused connection.
• AppKit's auto-injected "Window > New Tab" menu item → routes through the same override.
• Cmd+W still routes through closeTab() (regression check on performClose(_:)).

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