refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/risk_control/logstore/browser.go

@@ -0,0 +1,30 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package logstore
+
+import "strings"
+
+// ParseBrowserName performs lightweight User-Agent browser identification.
+func ParseBrowserName(ua string) string {
+	uaLower := strings.ToLower(ua)
+	if strings.Contains(uaLower, "micromessenger") {
+		return "WeChat"
+	}
+	if strings.Contains(uaLower, "postman") {
+		return "Postman"
+	}
+	if strings.Contains(uaLower, "edg/") || strings.Contains(uaLower, "edge") {
+		return "Edge"
+	}
+	if strings.Contains(uaLower, "firefox") {
+		return "Firefox"
+	}
+	if strings.Contains(uaLower, "chrome") {
+		return "Chrome"
+	}
+	if strings.Contains(uaLower, "safari") {
+		return "Safari"
+	}
+	return "Other"
+}