Unify fallback and unimplemented API error handling

- Add a shared common error response schema and unknown error code for business-level fallbacks
- Return business error payloads for router NoRoute and NoMethod fallbacks
- Align the existing event list fallback response with the shared error structure
- Introduce an explicit unimplemented API error type instead of relying on formatted strings
- Make /api batch dispatch return per-item status 600 error entries for unimplemented modules and actions without failing the whole batch

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/handler/api/subscenario/subscenario.go

@@ -1,7 +1,7 @@
 package subscenario
 
 import (
-	"fmt"
+	honokautils "honoka-chan/internal/utils"
 
 	"github.com/gin-gonic/gin"
 )
@@ -11,7 +11,7 @@ func SubscenarioApi(ctx *gin.Context, action string) (res any, err error) {
 	case "subscenarioStatus":
 		res, err = subscenarioStatus(ctx)
 	default:
-		err = fmt.Errorf("unimplemented action: subscenario: %s", action)
+		err = honokautils.NewUnimplementedActionError("subscenario", action)
 	}
 	return res, err
 }