feat(ui): storefront filters and admin category/tag management (#23)

Storefront gains a Filters drawer holding the category tree, colour-coded
tag pills, and a price range, with applied filters shown as removable
chips. Filter state lives in the URL query string, so a filtered view is
shareable and the back button works. Item cards now show their category
and tags.

Admin gains Categories and Tags tabs, and the item form gains a category
TreeSelect plus a tags Select that creates new tags on the fly.

The admin category tree tracks expansion in state rather than using
defaultExpandAll: that prop is evaluated once at mount, so a branch added
afterwards rendered collapsed and its children were unreachable. Creating
or moving a node now expands its parent. Caught by the new admin e2e spec.

The chip row is marked as a named group so its "Clear all" stays
distinguishable from the drawer's.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/styles.css

@@ -72,4 +72,46 @@ body { margin: 0; }
     padding-right: 10px;
     font-size: 13px;
   }
-}
+}
+/* Filter bar: the Filters button sits inline with the active-filter chips on
+   desktop; on a narrow screen the chips wrap onto their own line beneath it. */
+.filter-bar {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: 8px;
+  margin-bottom: 16px;
+}
+
+.active-filter-chips {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: 4px;
+}
+
+.active-filter-chips .ant-tag {
+  margin-inline-end: 0;
+}
+
+.item-category {
+  display: block;
+  font-size: 12px;
+  margin-bottom: 6px;
+}
+
+.item-tags {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 4px 0;
+  margin: 8px 0 4px;
+}
+
+@media (max-width: 575px) {
+  .filter-bar {
+    align-items: flex-start;
+  }
+  .filter-bar > .ant-btn {
+    width: 100%;
+  }
+}