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/main.tsx

@@ -1,4 +1,4 @@
-import React from 'react';
+import React, { useEffect, useState } from 'react';
 import ReactDOM from 'react-dom/client';
 import { BrowserRouter, Routes, Route } from 'react-router-dom';
 import { ConfigProvider, theme as antdTheme } from 'antd';
@@ -16,13 +16,35 @@ import { CartProvider } from './cart/CartContext';
 import { ThemeModeProvider, useThemeMode } from './theme/ThemeContext';
 import './styles.css';
 
+const REDUCED_MOTION_QUERY = '(prefers-reduced-motion: reduce)';
+
+// Respects the OS-level "reduce motion" accessibility setting by turning off
+// antd's transitions. Beyond the accessibility win, animated popups are a
+// standing source of flake in end-to-end tests, which drive the app with this
+// preference enabled.
+function usePrefersReducedMotion(): boolean {
+  const [prefers, setPrefers] = useState(
+    () => typeof window !== 'undefined' && window.matchMedia(REDUCED_MOTION_QUERY).matches
+  );
+
+  useEffect(() => {
+    const query = window.matchMedia(REDUCED_MOTION_QUERY);
+    const update = () => setPrefers(query.matches);
+    query.addEventListener('change', update);
+    return () => query.removeEventListener('change', update);
+  }, []);
+
+  return prefers;
+}
+
 function Root() {
   const { mode } = useThemeMode();
+  const prefersReducedMotion = usePrefersReducedMotion();
   return (
     <ConfigProvider
       theme={{
         algorithm: mode === 'dark' ? antdTheme.darkAlgorithm : antdTheme.defaultAlgorithm,
-        token: { colorPrimary: '#1a1a1a' }
+        token: { colorPrimary: '#1a1a1a', motion: !prefersReducedMotion }
       }}
     >
       <BrowserRouter>