feat(admin): add -urlPrefix flag for subdirectory deployment (#8670)

Allow the admin server to run behind a reverse proxy under a
subdirectory by adding a -urlPrefix flag (e.g. -urlPrefix=/seaweedfs).

Closes #8646

weed/admin/view/layout/context.go

@@ -1,12 +1,18 @@
 package layout
 
-import "net/http"
+import (
+	"net/http"
+
+	"github.com/a-h/templ"
+	"github.com/seaweedfs/seaweedfs/weed/admin/dash"
+)
 
 // ViewContext contains per-request metadata needed by layout templates.
 type ViewContext struct {
 	Request   *http.Request
 	Username  string
 	CSRFToken string
+	URLPrefix string
 }
 
 // NewViewContext builds a ViewContext from request metadata.
@@ -15,5 +21,11 @@ func NewViewContext(r *http.Request, username, csrfToken string) ViewContext {
 		Request:   r,
 		Username:  username,
 		CSRFToken: csrfToken,
+		URLPrefix: dash.URLPrefixFromContext(r.Context()),
 	}
 }
+
+// P returns the URL prefix prepended to the given path as a templ.SafeURL.
+func (v ViewContext) P(path string) templ.SafeURL {
+	return templ.SafeURL(v.URLPrefix + path)
+}