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/pages/block_types.ex

@@ -110,10 +110,27 @@ defmodule Berrypod.Pages.BlockTypes do
     },
     "newsletter_card" => %{
       name: "Newsletter signup",
-      description: "Email signup form for collecting subscriber addresses",
+      description: "Email signup form — currently decorative, does not collect emails",
       icon: "hero-envelope",
       allowed_on: :all,
-      settings_schema: []
+      hint:
+        "This block is decorative — form submissions aren't collected yet. Use it as a placeholder or remove it.",
+      settings_schema: [
+        %SettingsField{key: "title", label: "Title", type: :text, default: "Newsletter"},
+        %SettingsField{
+          key: "description",
+          label: "Description",
+          type: :textarea,
+          default:
+            "Sample newsletter signup. Replace with your own message to encourage subscribers."
+        },
+        %SettingsField{
+          key: "button_text",
+          label: "Button text",
+          type: :text,
+          default: "Subscribe"
+        }
+      ]
     },
     "social_links_card" => %{
       name: "Social links",