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/schema/common/common.go

@@ -1,7 +1,22 @@
 package commonschema
 
-import "honoka-chan/internal/constant"
+import (
+	"honoka-chan/internal/constant"
+)
 
 type ErrorData struct {
 	ErrorCode constant.ErrorCode `json:"error_code"`
 }
+
+type ErrorResp struct {
+	ResponseData ErrorData `json:"response_data"`
+	ReleaseInfo  []any     `json:"release_info"`
+	StatusCode   int       `json:"status_code"`
+}
+
+type ApiErrorResp struct {
+	Result     ErrorData `json:"result"`
+	Status     int       `json:"status"`
+	CommandNum bool      `json:"commandNum"`
+	TimeStamp  int64     `json:"timeStamp"`
+}