add no-JS progressive enhancement for all shop flows

Every key shop flow now works via plain HTML forms when JS is
unavailable. LiveView progressively enhances when JS connects.

- PDP: form wraps variant/qty/add-to-cart with action="/cart/add"
- Cart page: qty +/- and remove use form POST fallbacks
- Cart/search header icons are now links with phx-click enhancement
- Collection sort form has GET action + noscript submit button
- New /search page with form-based search for no-JS users
- CartController gains add/remove/update_item POST actions
- CartHook gains update_quantity_form and remove_item_form handlers
- Fix flaky analytics tests caused by event table pollution

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

assets/css/shop/components.css

@@ -462,6 +462,29 @@
     text-decoration: underline;
   }
 
+  .collection-sort-submit {
+    padding: 0.375rem 1rem;
+    font-size: var(--t-text-small);
+  }
+
+  /* ── Search page ── */
+
+  .search-page-form {
+    display: flex;
+    gap: 0.75rem;
+    margin-bottom: 2rem;
+    max-width: 32rem;
+  }
+
+  .search-page-form input {
+    flex: 1;
+  }
+
+  .search-page-count {
+    color: var(--t-text-secondary);
+    margin-bottom: 1.5rem;
+  }
+
   /* ── Breadcrumb ── */
 
   .breadcrumb {
@@ -1414,6 +1437,10 @@
     text-decoration: underline;
   }
 
+  .cart-remove-form {
+    display: contents;
+  }
+
   /* ── Cart empty state ── */
 
   .cart-empty {