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/view/layout/context.go

@@ -0,0 +1,19 @@
+package layout
+
+import "net/http"
+
+// ViewContext contains per-request metadata needed by layout templates.
+type ViewContext struct {
+	Request   *http.Request
+	Username  string
+	CSRFToken string
+}
+
+// NewViewContext builds a ViewContext from request metadata.
+func NewViewContext(r *http.Request, username, csrfToken string) ViewContext {
+	return ViewContext{
+		Request:   r,
+		Username:  username,
+		CSRFToken: csrfToken,
+	}
+}