T4.7: Remove forbidden platform imports from src/ui

src/ui now imports only src/app, src/domain, and Fyne.
Three violations were fixed:

- src/scheduler (NewRealClock): Service.Start() now creates the real
  clock internally; StartWith(clock) is the injectable seam for tests.
- src/platform/desktop (InstallDesktopIntegration): moved into
  Service.InstallDesktopIcon in new src/app/platform.go.
- src/platform/autostart (AutostartStatus, SetAutostart): moved into
  Service.AutostartStatus and Service.ApplyAutostart in platform.go.

go test -race ./... green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/service.go

@@ -91,11 +91,16 @@ func NewService(store *storage.Store, jobs []domain.Job) *Service {
 	return s
 }
 
-// Start begins scheduling. It installs a cancelable run context and a timing
-// loop driven by the given clock; every tick calls RunDue. Pass
-// scheduler.NewRealClock() in production. Start is expected once, during setup,
-// before any concurrent use.
-func (s *Service) Start(clock scheduler.Clock) {
+// Start begins scheduling with the real wall clock. It is the production entry
+// point; tests should call StartWith and supply a fake clock instead. Start is
+// expected once, during setup, before any concurrent use.
+func (s *Service) Start() {
+	s.StartWith(scheduler.NewRealClock())
+}
+
+// StartWith begins scheduling driven by the given clock; every tick calls
+// RunDue. Used by tests to inject a fake clock.
+func (s *Service) StartWith(clock scheduler.Clock) {
 	s.mu.Lock()
 	ctx, cancel := context.WithCancel(context.Background())
 	s.ctx = ctx