fix(04-WR-03): trim whitespace from description in TaskUpdateHandler

A description of spaces-only was being stored as a valid non-null DB value
because the empty-string check ran before trimming. Now consistent with how
other nullable text fields are handled.

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

backend/internal/web/handlers_tasks.go

@@ -333,7 +333,7 @@ func TaskUpdateHandler(deps TasksDeps) http.HandlerFunc {
 		}
 
 		title := strings.TrimSpace(r.PostFormValue("title"))
-		description := r.PostFormValue("description")
+		description := strings.TrimSpace(r.PostFormValue("description"))
 
 		var errs templates.TaskUpdateErrors
 		if title == "" {