Update `llama-server` README secion about WebUI

[jakexcosme]

Note: This issue was copied from ggml-org#16256

Original Author: @allozaur
Original Issue Number: ggml-org#16256
Created: 2025-09-25T14:55:41Z

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After recent refactor, redesign & migration of WebUI to Svelte (ggml-org#14839) README needs to be updated