fix(04-WR-02): set HX-Retarget/HX-Reswap on 422 path in TaskCreateHandler

Without these headers, HTMX used the form's own hx-target="#column-{status}"
+ hx-swap="beforeend", appending the error form into the task column and
destroying all visible task cards. The error form now lands back in the
add-task slot where it belongs.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

backend/internal/web/handlers_tasks.go

@@ -147,6 +147,10 @@ func TaskCreateHandler(deps TasksDeps) http.HandlerFunc {
 
 		if errs.Title != "" {
 			w.Header().Set("Content-Type", "text/html; charset=utf-8")
+			if r.Header.Get("HX-Request") == "true" {
+				w.Header().Set("HX-Retarget", "#add-task-slot-"+statusStr)
+				w.Header().Set("HX-Reswap", "innerHTML")
+			}
 			w.WriteHeader(http.StatusUnprocessableEntity)
 			if r.Header.Get("HX-Request") == "true" {
 				_ = templates.TaskCreateFormFragment(