Show quit confirmation dialog when sites are running

[bcotrim]

Related issues

• Fixes STU-1112

Proposed Changes

• Add quit confirmation dialog when user quits Studio with running sites
• Dialog offers two options: "Stop sites" or "Leave running"
• "Remember my choice" checkbox saves preference to skip dialog on future quits
• Store preference as stopSitesOnQuit boolean in user data (undefined = show dialog, true = stop, false = leave running)
• Add getRunningSiteCount() helper function to track running sites
• Conditionally stop servers on quit based on user preference

Testing Instructions

Happy Path

• Start one or more sites, then quit Studio (Cmd+Q or close window)
• Verify modal appears with "Stop sites" and "Leave running" buttons
• Click "Stop sites" → verify sites stop, then app quits
• Click "Leave running" → verify app quits but sites continue running (check with studio site list)

Remember Choice

• Start a site, quit Studio, check "Remember my choice", click "Stop sites"
• Start a site again, quit Studio → verify no dialog appears and sites stop
• To reset: manually delete stopSitesOnQuit from ~/Library/Application Support/WordPress Studio/appdata-v1.json

Edge Cases

• Quit with no running sites → no dialog should appear
• CMD+Q keyboard shortcut respects modal behavior

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?