fix: prevent connection data loss on normal app quit

[datlechin]

Summary

Fixes #452 — saved connections disappear after normal app quit (Cmd+Q) but persist after force quit.

• Root cause: WelcomeWindowView.loadConnections() overwrites UserDefaults with an empty array when storage.loadConnections() returns empty during SwiftUI scene teardown on quit. Since DatabaseConnection.sampleConnections was [], this destructively erased valid saved data.
• Remove the destructive overwrite — loadConnections() now simply reads from storage without writing back
• Add UserDefaults.standard.synchronize() in applicationWillTerminate as a defensive flush
• Replace broken sampleConnections (empty array indexed at [0] in previews) with a proper DatabaseConnection.preview static

Test plan

• Create a new connection and connect successfully
• Quit app normally (Cmd+Q), reopen — verify connection is still listed
• Force quit app, reopen — verify connection is still listed
• Fresh install (no saved connections) — verify welcome window shows empty state without errors