feat: add user-facing deployment cancel endpoint

Add POST /apps/{id}/deployments/cancel endpoint that allows users to
cancel in-progress deployments via the web UI.

Changes:
- Add CancelDeploy() and HasActiveDeploy() public methods to deploy service
- Add HandleCancelDeploy handler
- Wire route in routes.go
- Add cancel button to app detail template (shown during active deployments)
- Add handler tests for cancel endpoint

fixes #66

internal/server/routes.go

@@ -68,6 +68,7 @@ func (s *Server) SetupRoutes() {
 		r.Post("/apps/{id}", s.handlers.HandleAppUpdate())
 		r.Post("/apps/{id}/delete", s.handlers.HandleAppDelete())
 		r.Post("/apps/{id}/deploy", s.handlers.HandleAppDeploy())
+		r.Post("/apps/{id}/deployments/cancel", s.handlers.HandleCancelDeploy())
 		r.Get("/apps/{id}/deployments", s.handlers.HandleAppDeployments())
 		r.Get("/apps/{id}/deployments/{deploymentID}/logs", s.handlers.HandleDeploymentLogsAPI())
 		r.Get("/apps/{id}/deployments/{deploymentID}/download", s.handlers.HandleDeploymentLogDownload())