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/tests/e2e/admin-inline-category.spec.ts

@@ -0,0 +1,77 @@
+import { test, expect } from '@playwright/test';
+
+const suffix = () => `i${Date.now().toString(36)}${Math.random().toString(36).slice(2, 7)}`;
+
+// Opening the form kicks off a categories fetch. Interacting with the category
+// field before it lands means the tree re-renders under the cursor, so wait for
+// the data rather than racing it.
+async function openItemForm(page: import('@playwright/test').Page) {
+  // Opening the form fetches both categories and tags, and each one re-renders
+  // the modal as it lands. Waiting for only one still leaves the second to
+  // reflow the popup mid-interaction.
+  const loaded = Promise.all([
+    page.waitForResponse((res) => res.url().includes('/api/admin/categories') && res.request().method() === 'GET'),
+    page.waitForResponse((res) => res.url().includes('/api/admin/tags') && res.request().method() === 'GET')
+  ]);
+  await page.getByRole('button', { name: 'Add Item' }).click();
+  await loaded;
+}
+
+test.describe('Inline category creation from the item form', () => {
+  test('creates a category without leaving the item form and assigns it', async ({ page }) => {
+    const RUN = suffix();
+    const categoryName = `Inline ${RUN}`;
+    const itemName = `Item ${RUN}`;
+
+    await page.goto('/admin');
+    await openItemForm(page);
+    await page.getByLabel('Name').fill(itemName);
+    await page.getByLabel('Price (USD)').fill('99');
+
+    await page.getByRole('dialog').getByLabel('Category', { exact: true }).click();
+    const nameInput = page.getByPlaceholder('New category name');
+    await expect(nameInput).toBeVisible();
+    await expect(page.getByRole('button', { name: 'Create category' })).toBeVisible();
+    await nameInput.fill(categoryName);
+    // Submitted with Enter rather than a click: the popup sits over a
+    // virtualized tree that keeps re-measuring, so a click target inside it is
+    // never geometrically stable. Enter runs the same handler as the button.
+    await nameInput.press('Enter');
+
+    // The new category should be selected straight away — having to hunt for it
+    // in the tree afterwards defeats the point of creating it inline.
+    await expect(page.getByRole('dialog').getByText(categoryName)).toBeVisible();
+
+    await page.getByRole('button', { name: 'OK' }).click();
+    await expect(page.getByText('Item added')).toBeVisible();
+
+    const items = await (await page.request.get('/api/admin/items')).json();
+    const saved = items.find((item: { name: string }) => item.name === itemName);
+    expect(saved).toBeTruthy();
+    expect(saved.category_name).toBe(categoryName);
+  });
+
+  test('reports a duplicate category name instead of silently doing nothing', async ({ page }) => {
+    const RUN = suffix();
+    const categoryName = `Dupe ${RUN}`;
+
+    const created = await page.request.post('/api/admin/categories', {
+      data: { name: categoryName, parent_id: null }
+    });
+    expect(created.status()).toBe(201);
+
+    await page.goto('/admin');
+    await openItemForm(page);
+    await page.getByRole('dialog').getByLabel('Category', { exact: true }).click();
+    const nameInput = page.getByPlaceholder('New category name');
+    await expect(nameInput).toBeVisible();
+    await expect(page.getByRole('button', { name: 'Create category' })).toBeVisible();
+    await nameInput.fill(categoryName);
+    // Submitted with Enter rather than a click: the popup sits over a
+    // virtualized tree that keeps re-measuring, so a click target inside it is
+    // never geometrically stable. Enter runs the same handler as the button.
+    await nameInput.press('Enter');
+
+    await expect(page.getByText(/already exists/i)).toBeVisible();
+  });
+});