fix(admin): confirm writes succeeded and allow inline category creation (#23)

Reported from testing: the item form said "Item added" for a save that
never happened.

saveItem and the other admin calls returned res.json() without checking
res.ok, so a 4xx/5xx resolved normally and every caller reported success
for a write the server had rejected. That is worse than failing outright,
because nothing prompts the user to look for the missing row. All admin
calls now throw on a non-OK response, and the handlers report the error,
keep the form open so entered values survive, and only claim success once
the server has accepted the write. Mark sold/available previously did
nothing visible on failure at all.

Categories can now be created from the item form, as tags already could.
Previously a category that did not exist yet meant abandoning a
half-filled form for the Categories tab. New categories are created at
the top level; nesting stays in the Categories tab.

The control lives in its own component: inline, every keystroke
re-rendered the whole Inventory component and rebuilt the category tree,
which visibly jittered the open popup. It sits above the tree rather than
below it, where a long list both hid it and made its position depend on
the list's measured height. The tree no longer expands everything on
open, which does not scale past a screenful; it has search instead.

The app now honours prefers-reduced-motion by disabling antd transitions,
and the e2e suite runs with that preference set.

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

frontend/src/api.ts

@@ -67,32 +67,54 @@ export async function fetchFilterOptions(): Promise<FilterOptions> {
   return res.json();
 }
 
+// Every admin call goes through this. Without the res.ok check a 4xx/5xx still
+// resolves — the caller then reports success for a write that never happened,
+// which is worse than failing outright because nothing prompts the user to look
+// for the missing row.
+async function expectOk(res: Response, action: string): Promise<Response> {
+  if (res.ok) return res;
+  const detail = await res.json().catch(() => null);
+  throw new Error(detail?.error ? `${action}: ${detail.error}` : action);
+}
+
 export async function fetchAdminItems(): Promise<Item[]> {
-  const res = await fetch('/api/admin/items');
+  const res = await expectOk(await fetch('/api/admin/items'), 'failed to load items');
   return res.json();
 }
 
 export async function saveItem(id: number | null, formData: FormData): Promise<Item> {
   const url = id ? `/api/admin/items/${id}` : '/api/admin/items';
-  const res = await fetch(url, { method: id ? 'PUT' : 'POST', body: formData });
+  const res = await expectOk(
+    await fetch(url, { method: id ? 'PUT' : 'POST', body: formData }),
+    'failed to save item'
+  );
   return res.json();
 }
 
 export async function deleteItem(id: number): Promise<void> {
-  await fetch(`/api/admin/items/${id}`, { method: 'DELETE' });
+  await expectOk(await fetch(`/api/admin/items/${id}`, { method: 'DELETE' }), 'failed to delete item');
 }
 
 export async function deleteItemImage(itemId: number, imageId: number): Promise<void> {
-  await fetch(`/api/admin/items/${itemId}/images/${imageId}`, { method: 'DELETE' });
+  await expectOk(
+    await fetch(`/api/admin/items/${itemId}/images/${imageId}`, { method: 'DELETE' }),
+    'failed to remove image'
+  );
 }
 
 export async function markSold(id: number): Promise<Item> {
-  const res = await fetch(`/api/admin/items/${id}/mark-sold`, { method: 'POST' });
+  const res = await expectOk(
+    await fetch(`/api/admin/items/${id}/mark-sold`, { method: 'POST' }),
+    'failed to mark sold'
+  );
   return res.json();
 }
 
 export async function markAvailable(id: number): Promise<Item> {
-  const res = await fetch(`/api/admin/items/${id}/mark-available`, { method: 'POST' });
+  const res = await expectOk(
+    await fetch(`/api/admin/items/${id}/mark-available`, { method: 'POST' }),
+    'failed to mark available'
+  );
   return res.json();
 }