Update eca-webview: tighten empty-state layout

Bump eca-webview to b9f1069. Two empty-state polish fixes that both
matter for eca-web:

* ChatPrompt.scss: drop the 0.3em prompt bottom margin on non-desktop
  hosts. Particularly visible on mobile web.eca.dev where every pixel
  of viewport height counts and the surface already supplies framing.
* Chat.scss: restore the centered-hero welcome layout in hero mode
  (which fires on web.eca.dev empty chats since `isWeb` is true for
  `data-editor="web"`). A recent `.messages-wrapper` insertion silently
  broke the `>` direct-child selectors that anchored the welcome to
  the bottom of the messages region and added a balancing `::after`
  spacer; walking through the wrapper restores welcome+prompt as a
  centered group with empty space top and bottom.

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Co-Authored-By: eca-agent <git@eca.dev>

Author: ericdallo