add launch-at-login setting via SMAppService

Checkbox registers/unregisters SMAppService.mainApp (macOS 13+ API, no
helper app needed). Deliberately not mirrored in UserDefaults: the
service status is the single source of truth, and syncUI reads it live
so changes made in System Settings > Login Items stay in sync. Failures
revert the checkbox and surface the error in an alert.

Note: registration points at the app bundle's current path, so it only
behaves properly once BrewBar runs from a stable location like
/Applications — debug builds may register the build folder copy.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

BrewBar/SettingsWindowController.swift

@@ -1,4 +1,5 @@
 import Cocoa
+import ServiceManagement
 
 class SettingsWindowController: NSWindowController {
     weak var appDelegate: AppDelegate?
@@ -13,6 +14,7 @@ class SettingsWindowController: NSWindowController {
 
     var intervalPopup: NSPopUpButton!
     var customMinutesField: NSTextField!
+    var launchAtLoginCheckbox: NSButton!
 
     convenience init(appDelegate: AppDelegate) {
         let window = NSWindow(
@@ -66,9 +68,17 @@ class SettingsWindowController: NSWindowController {
         ])
         customRow.orientation = .horizontal
 
+        launchAtLoginCheckbox = NSButton(
+            checkboxWithTitle: "Launch BrewBar at login",
+            target: self,
+            action: #selector(toggleLaunchAtLogin(_:))
+        )
+
         let stack = NSStackView(views: [
             frequencyRow,
             customRow,
+            separator(),
+            launchAtLoginCheckbox,
         ])
         stack.orientation = .vertical
         stack.alignment = .leading
@@ -99,6 +109,16 @@ class SettingsWindowController: NSWindowController {
             intervalPopup.selectItem(at: presets.count) // Custom
         }
         customMinutesField.stringValue = "\(Int(interval / 60))"
+
+        // SMAppService is the source of truth — the user may have changed
+        // this in System Settings > Login Items while we weren't looking
+        launchAtLoginCheckbox.state = SMAppService.mainApp.status == .enabled ? .on : .off
+    }
+
+    func separator() -> NSBox {
+        let box = NSBox()
+        box.boxType = .separator
+        return box
     }
 
     // MARK: - Actions
@@ -124,4 +144,22 @@ class SettingsWindowController: NSWindowController {
         appDelegate?.restartTimer()
         syncUI()
     }
+
+    @objc func toggleLaunchAtLogin(_ sender: NSButton) {
+        do {
+            if sender.state == .on {
+                try SMAppService.mainApp.register()
+            } else {
+                try SMAppService.mainApp.unregister()
+            }
+        } catch {
+            sender.state = sender.state == .on ? .off : .on // revert on failure
+
+            let alert = NSAlert()
+            alert.alertStyle = .warning
+            alert.messageText = "Could not change login item"
+            alert.informativeText = error.localizedDescription
+            alert.runModal()
+        }
+    }
 }