refactor: replace NotificationCenter panel sync with shared Observable

[datlechin]

Summary

• Extract panel visibility (isPresented, panelWidth) from RightPanelState into RightPanelVisibility — a shared @Observable singleton
• All windows observe the same instance, so panel state syncs automatically without NotificationCenter
• Removes: NotificationCenter observer/poster, isSyncing boolean gate, manual observer cleanup in teardown()/deinit, UserDefaults-as-message-bus pattern
• RightPanelState retains per-window state: editState, aiViewModel, activeTab, onSave
• Net -53 lines

Test plan

• Toggle right panel in one window — all windows toggle together
• Drag panel resize handle — width persists across app restart
• Close and reopen app — panel visibility state restored
• Open AI chat panel via error dialog — right panel opens correctly