Remove cross-domain template dependencies from shared infrastructure

- macros/search.html: shared search input macros (mobile + desktop)
- macros/cart_icon.html: shared cart icon/badge macro (count param, no DB)
- macros/layout.html: inline hamburger icon, use shared search macro
- _oob.html: use cart_mini_html fragment slot instead of cart template import
- db/session.py: guard teardown rollback against committed/dead sessions

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

browser/templates/macros/cart_icon.html

@@ -0,0 +1,31 @@
+{# Cart icon/badge — shows logo when empty, cart icon with count when items present #}
+
+{% macro cart_icon(count=0, oob=False) %}
+<div id="cart-mini" {% if oob %}hx-swap-oob="{{oob}}"{% endif %}>
+  {% if count == 0 %}
+  <div class="h-12 w-12 rounded-full overflow-hidden border border-stone-300 flex-shrink-0">
+    <a
+      href="{{ blog_url('/') }}"
+      class="h-full w-full font-bold text-5xl flex-shrink-0 flex flex-row items-center gap-1"
+    >
+      <img
+        src="{{ site().logo }}"
+        class="h-full w-full rounded-full object-cover border border-stone-300 flex-shrink-0"
+      >
+    </a>
+  </div>
+  {% else %}
+    <a
+      href="{{ cart_url('/') }}"
+      class="relative inline-flex items-center justify-center text-stone-700 hover:text-emerald-700"
+    >
+      <i class="fa fa-shopping-cart text-5xl" aria-hidden="true"></i>
+      <span
+        class="absolute top-1/2 left-1/2 -translate-x-1/2 -translate-y-1/2 inline-flex items-center justify-center rounded-full bg-emerald-600 text-white text-sm w-5 h-5"
+      >
+        {{ count }}
+      </span>
+    </a>
+  {% endif %}
+</div>
+{% endmacro %}