fix(admin): theme, American English, and inventory/reservation tooling (#27)

Seven reported items, of which the first four had two root causes.

The active tab was invisible in dark mode because colorPrimary was
hardcoded to #1a1a1a in both themes. The accent now inverts with the
theme, and colorTextLightSolid inverts with it, or a near-white accent
would get antd's default white label and disappear.

The Category tab, Tag tab, and item-form category selector ignored the
theme entirely. antd declares main: lib/index.js and module: es/index.js,
so importing from 'antd' resolves to the ES build while 'antd/lib/...'
loads the CommonJS one — two copies, two React contexts, and no
ConfigProvider for anything deep-imported. Switching those files to
antd/es/* keeps the deep-import convention and shares the instance. This
was introduced by my own use of the lib path; es is correct under Vite.
Two storefront components had the same latent bug.

"Colour" is now "Color".

The Customers tab shows how many items each customer is holding, as a
link opening the item list with a Release button. Release mirrors the
customer's own cart removal — drop the cart row, return the item to
available, guarded on 'reserved' so it can never resurrect a sold item —
and deliberately sends no email about an action the customer did not
take. The count is a subquery rather than another join, which would have
multiplied rows and inflated order_count and total_spent_cents.

The Inventory tab filters by category, tags, price, and status, reusing
the storefront's parser and query builder so the two cannot drift.
Reserved is one option in a Status filter rather than a standalone toggle.

Also fixes two defects the screenshots exposed: the reserved-count link
bubbled to the row handler and opened the customer drawer behind the
dialog, and .admin-category-node had no CSS at all, so the tree node name,
item count, and actions ran together as one string.

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

frontend/src/styles.css

@@ -115,3 +115,30 @@ body { margin: 0; }
     width: 100%;
   }
 }
+
+/* Admin inventory filter row — always visible above the table, wrapping onto
+   further lines on narrow screens rather than scrolling horizontally. */
+.inventory-filters {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: 8px;
+  margin-bottom: 16px;
+}
+
+/* Admin category tree node: name, item count, then actions pushed to the
+   right. Without this the three run together as one unbroken string. */
+.admin-category-node {
+  display: inline-flex;
+  align-items: center;
+  gap: 12px;
+  width: 100%;
+}
+
+.admin-category-node > span:first-child {
+  font-weight: 500;
+}
+
+.admin-category-node .ant-space {
+  margin-left: auto;
+}