Add configuration UI for headless frontends
Canvas Headless currently allows configuring a single frontend URL with no UI. Expand it to allow multiple frontend URLs, and provide a UI inside Canvas that also shows setup steps. ![kép](/uploads/41bc9d2ce9634dd5443bb7419dd60e7a/kép.png){width=745 height=600} ![kép](/uploads/bb9f50613cf4340fdb8e049b93efc2b1/kép.png){width=745 height=600} ![kép](/uploads/e9ffd96f50244c17b4fc662066c009f5/kép.png){width=745 height=600} ![kép](/uploads/8c5161d0c539a1f3dc9626e5964fd9d5/kép.png){width=495 height=166}
issue