add image picker to page editor and fix thumbnail layout

Wire up image field in block settings with a modal picker that
browses the media library. Fix picker thumbnails collapsing to
14px by replacing overflow:hidden with overflow:clip on grid
items (hidden sets min-height:0 in grid context). Polish media
library mobile sheet with scrim overlay and tighter spacing.

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

lib/berrypod_web/live/admin/media.ex

@@ -330,6 +330,7 @@ defmodule BerrypodWeb.Admin.Media do
         </div>
 
         <%!-- detail panel --%>
+        <div :if={@selected_image} class="media-detail-scrim" phx-click="deselect_image"></div>
         <aside :if={@selected_image} class="media-detail">
           <div class="media-detail-header">
             <h3>Image details</h3>