fix(constants): align dev BackendVersion default with shipped frontend

The default was stale at 4.14.0 while frontend/package.json and the
embedded statics build at 4.19.1, producing a startup version-mismatch
warning and wrong /site/ping output in dev builds. Release builds still
inject the tag via ldflags.

Authored By: TDvorak <info@tdvorak.dev>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

application/constants/constants.go

@@ -3,7 +3,7 @@ package constants
 // These values will be injected at build time, DO NOT EDIT.
 
 // BackendVersion 当前后端版本号
-var BackendVersion = "4.14.0"
+var BackendVersion = "4.19.1"
 
 // IsPro 是否为Pro版本
 var IsPro = "false"