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/router/router.go

@@ -2,6 +2,9 @@
 package router
 
 import (
+	"honoka-chan/internal/constant"
+	commonschema "honoka-chan/internal/schema/common"
+	"honoka-chan/internal/session"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -104,6 +107,21 @@ func SifRouter(r *gin.Engine) {
 		ctx.HTML(http.StatusOK, "common/manga.html", gin.H{})
 	})
 
+	notFoundHandler := func(ctx *gin.Context) {
+		ss := session.Attach(ctx)
+		defer ss.Finalize()
+
+		ss.Respond(commonschema.ErrorResp{
+			ResponseData: commonschema.ErrorData{
+				ErrorCode: constant.ErrorCodeUnknown,
+			},
+			ReleaseInfo: []any{},
+			StatusCode:  600,
+		})
+	}
+	r.NoRoute(notFoundHandler)
+	r.NoMethod(notFoundHandler)
+
 	// router
 	for groupPath, groupInfo := range groups {
 		groupApi := r.Group(groupPath, groupInfo.InitialHandlers...)