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/navigation/navigation.go

@@ -1,7 +1,7 @@
 package navigation
 
 import (
-	"fmt"
+	honokautils "honoka-chan/internal/utils"
 )
 
 func NavigationApi(action string) (res any, err error) {
@@ -9,7 +9,7 @@ func NavigationApi(action string) (res any, err error) {
 	case "specialCutin":
 		res, err = SpecialCutin()
 	default:
-		err = fmt.Errorf("unimplemented action: navigation: %s", action)
+		err = honokautils.NewUnimplementedActionError("navigation", action)
 	}
 	return res, err
 }