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/banner/banner.go

@@ -1,13 +1,13 @@
 package banner
 
-import "fmt"
+import honokautils "honoka-chan/internal/utils"
 
 func BannerApi(action string) (res any, err error) {
 	switch action {
 	case "bannerList":
 		res, err = bannerList()
 	default:
-		err = fmt.Errorf("unimplemented action: banner: %s", action)
+		err = honokautils.NewUnimplementedActionError("banner", action)
 	}
 	return res, err
 }