Admin UI: replace gin with mux (#8420)

* Replace admin gin router with mux

* Update layout_templ.go

* Harden admin handlers

* Add login CSRF handling

* Fix filer copy naming conflict

* address comments

* address comments

weed/admin/dash/context.go

@@ -0,0 +1,58 @@
+package dash
+
+import "context"
+
+type contextKey string
+
+const (
+	contextUsernameKey contextKey = "admin.username"
+	contextRoleKey     contextKey = "admin.role"
+	contextCSRFKey     contextKey = "admin.csrf"
+)
+
+// WithAuthContext stores auth metadata on the request context.
+func WithAuthContext(ctx context.Context, username, role, csrfToken string) context.Context {
+	if username != "" {
+		ctx = context.WithValue(ctx, contextUsernameKey, username)
+	}
+	if role != "" {
+		ctx = context.WithValue(ctx, contextRoleKey, role)
+	}
+	if csrfToken != "" {
+		ctx = context.WithValue(ctx, contextCSRFKey, csrfToken)
+	}
+	return ctx
+}
+
+// UsernameFromContext retrieves the username from context.
+func UsernameFromContext(ctx context.Context) string {
+	if ctx == nil {
+		return ""
+	}
+	if value, ok := ctx.Value(contextUsernameKey).(string); ok {
+		return value
+	}
+	return ""
+}
+
+// RoleFromContext retrieves the role from context.
+func RoleFromContext(ctx context.Context) string {
+	if ctx == nil {
+		return ""
+	}
+	if value, ok := ctx.Value(contextRoleKey).(string); ok {
+		return value
+	}
+	return ""
+}
+
+// CSRFTokenFromContext retrieves the CSRF token from context.
+func CSRFTokenFromContext(ctx context.Context) string {
+	if ctx == nil {
+		return ""
+	}
+	if value, ok := ctx.Value(contextCSRFKey).(string); ok {
+		return value
+	}
+	return ""
+}