add block previews, picker thumbnails and newsletter settings

Block cards now show a one-line content summary below the name.
Block picker items include SVG wireframe thumbnails. Newsletter
block marked as decorative with configurable title/description
and form submission prevented on the shop side.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod_web/page_renderer.ex

@@ -303,7 +303,15 @@ defmodule BerrypodWeb.PageRenderer do
   end
 
   defp render_block(%{block: %{"type" => "newsletter_card"}} = assigns) do
-    ~H"<.newsletter_card />"
+    settings = assigns.block["settings"] || %{}
+
+    assigns =
+      assigns
+      |> assign(:title, settings["title"] || "Newsletter")
+      |> assign(:description, settings["description"] || "")
+      |> assign(:button_text, settings["button_text"] || "Subscribe")
+
+    ~H"<.newsletter_card title={@title} description={@description} button_text={@button_text} />"
   end
 
   defp render_block(%{block: %{"type" => "social_links_card"}} = assigns) do