fix search modal closing on keypress and add admin header icon

Track search_open as server state so morphdom doesn't reset display
to none on re-render. Move admin bar from layout banner to a gear
icon in the header actions. Extract layout_assigns/1 helper so page
templates use a spread instead of listing every attr explicitly.

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

lib/simpleshop_theme_web/components/layouts/shop.html.heex

@@ -1,13 +1,2 @@
-<div
-  :if={assigns[:current_scope]}
-  style="background-color: var(--t-surface-raised, #f5f5f5); border-bottom: 1px solid var(--t-border-default, #e5e5e5); padding: 0.25rem 1rem; font-size: 0.75rem; text-align: right;"
->
-  <.link
-    href={~p"/admin"}
-    style="color: var(--t-text-secondary, #666); text-decoration: none;"
-  >
-    Admin
-  </.link>
-</div>
 <.shop_flash_group flash={@flash} />
 {@inner_content}