ui: add launch-at-login (autostart) toggle for the UI

Add an Autostart Wails service wrapping app.Autostart and a toggle in
the General settings tab. The OS login-item registration is the single
source of truth (nothing mirrored to the preferences file). Affects the
graphical UI only, not the daemon. The toggle hides itself on platforms
where autostart is unsupported.

client/ui/services/autostart.go

@@ -0,0 +1,62 @@
+//go:build !android && !ios && !freebsd && !js
+
+package services
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/wailsapp/wails/v3/pkg/application"
+)
+
+// Autostart is the Wails-bound facade over Wails' AutostartManager. The OS
+// login-item registration (launchd/SMAppService on macOS, HKCU\…\Run on
+// Windows, an XDG .desktop on Linux) is the single source of truth — IsEnabled
+// reads it directly, so nothing is mirrored to the preferences file. Enable
+// registers the running executable to launch at login with no extra arguments;
+// the app comes up hidden into the tray, same as a normal launch.
+type Autostart struct {
+	mgr *application.AutostartManager
+}
+
+// NewAutostart wraps the application's AutostartManager (app.Autostart).
+func NewAutostart(mgr *application.AutostartManager) *Autostart {
+	return &Autostart{mgr: mgr}
+}
+
+// Supported reports whether autostart can be toggled on this platform. The
+// frontend hides the toggle entirely when this is false.
+func (a *Autostart) Supported(_ context.Context) bool {
+	_, err := a.mgr.Status()
+	return !errors.Is(err, application.ErrAutostartNotSupported)
+}
+
+// IsEnabled reports whether the app is currently registered to launch at
+// login. On an unsupported platform it returns false without error so the
+// frontend can render the toggle off (gated by Supported).
+func (a *Autostart) IsEnabled(_ context.Context) (bool, error) {
+	enabled, err := a.mgr.IsEnabled()
+	if err != nil {
+		if errors.Is(err, application.ErrAutostartNotSupported) {
+			return false, nil
+		}
+		return false, fmt.Errorf("read autostart state: %w", err)
+	}
+	return enabled, nil
+}
+
+// SetEnabled registers (enabled) or removes (disabled) the launch-at-login
+// entry. The change takes effect on the next login, not immediately.
+func (a *Autostart) SetEnabled(_ context.Context, enabled bool) error {
+	if enabled {
+		if err := a.mgr.Enable(); err != nil {
+			return fmt.Errorf("enable autostart: %w", err)
+		}
+		return nil
+	}
+	if err := a.mgr.Disable(); err != nil {
+		return fmt.Errorf("disable autostart: %w", err)
+	}
+	return nil
+}