refactor: migrate 5 settings notifications to AppEvents

[datlechin]

Summary

Phase 5 tail. Five settings notifications moved off NotificationCenter onto typed AppEvents publishers (Void payload):

• editorSettingsChanged
• dataGridSettingsChanged
• aiSettingsChanged
• terminalSettingsChanged
• accessibilityTextSizeChanged

7 post sites and 7 observer sites migrated. SettingsNotifications.swift and the AppSettingsManager.notifyChange helper deleted.

Changed

• 1 post: ThemeEngine.swift accessibilityTextSize forwarder
• 5 posts: AppSettingsManager didSet hooks and accessibility observer
• 5 observers: TerminalSessionState, DataGridCoordinator, DataGridCellRegistry, SQLEditorCoordinator (editor + ai), SQLEditorView
• AppEvents.swift gains 5 publishers

NSObjectProtocol observers replaced with AnyCancellable. Manual removeObserver in deinit removed where the observer was the only reason to keep the override.

Test plan

• Build clean
• swiftlint clean (verified, 0 violations on touched files)
• Toggle editor settings (line numbers, vim, font), confirm SQLEditor reacts
• Toggle data grid row height, confirm grid re-tiles
• Toggle terminal font, confirm terminal updates
• Toggle AI provider, confirm inline suggestions source switches
• System Settings > Accessibility text size change, confirm editor font reloads

Punch list status

Bucket	Migrated	Remaining
Tier 1 (high traffic)	2 / 8	6
Tier 2 (single observer)	0 / 10	10
Total domain notifs	7 / 35	28

Tier 3 command-style notifications stay on NotificationCenter until the command-bus pass.