fix: auto-focus SQL editor on new tab and restore cursor on tab switch

[datlechin]

Summary

• Auto-focus the SQL editor when opening a new query tab so users can type immediately without clicking
• Restore cursor blink when switching between editor tabs

Root Cause

• No auto-focus: prepareCoordinator never called makeFirstResponder on the editor's text view, and SourceEditorState() initializes with nil cursor positions — so even with focus, insertText had no insertion point
• Missing cursor on tab switch: resignKeyWindow calls removeCursors() destroying cursor subviews, but becomeKeyWindow only resets the blink timer without recreating them

Changes

SQLEditorCoordinator.swift:

• In prepareCoordinator: call makeFirstResponder then setCursorPositions (order matters — cursor view creation guards on isFirstResponder)
• Add NSWindow.didBecomeKeyNotification observer to re-apply cursor positions when window regains key status

Test plan

• Open a connection → first tab editor is auto-focused with blinking cursor, can type immediately
• Cmd+T to open second tab → editor is auto-focused
• Switch between tabs → cursor blinks correctly on each tab
• Typing works on all tabs without clicking first