refactor: restructure settings for correct data flow and native macOS patterns

[datlechin]

Summary

• SyncSettings integrated into AppSettingsManager — was bypassing the central manager with local @State and direct AppSettingsStorage calls, now uses the same didSet → persist + SyncChangeTracker.markDirty pattern as all other settings domains
• GeneralSettingsView binding honesty — removed hidden @Bindable settingsManager dependency; now receives @Binding var tabSettings and onResetAll closure explicitly
• Replaced private ObjC selector — Selector(("showSettingsWindow:")) (3 call sites) replaced with Environment(\.openSettings) (macOS 14+ public API) and NotificationCenter for AppKit contexts
• HistorySettings defensive decoding — added init(from decoder:) with decodeIfPresent + defaults, matching all other settings structs
• Single theme activation path — removed redundant .onChange in ThemeListView that duplicated activation already handled by AppSettingsManager.appearance.didSet

Test plan

• Open Settings (Cmd+,), verify all 10 tabs render correctly
• Change settings in each tab, restart app, verify persistence
• Toggle iCloud Sync on/off, verify SyncCoordinator receives enable/disable calls
• Click SyncStatusIndicator → verify Settings opens to Sync tab
• Drag-install a plugin → verify Settings opens to Plugins tab
• Switch themes in Appearance tab → verify theme activates once (not twice)
• Click "Reset All Settings to Defaults" in General tab → verify all settings reset
• Build with xcodebuild — no warnings in changed files