feat(06-01): add TemplatePage, routing, sidebar nav, and i18n translations

- Create TemplatePage with add form (category/tier/amount), items table, and empty state
- Add /template route to App.tsx with TemplatePage component
- Add Template nav item to sidebar (between Categories and Settings)
- Add template and nav.template i18n keys for EN and DE
- Fix unused import in useTemplate hook

frontend/src/components/AppLayout.tsx

@@ -1,6 +1,6 @@
 import { useTranslation } from 'react-i18next'
 import { Link, useLocation } from 'react-router-dom'
-import { LayoutDashboard, Tags, Settings, LogOut } from 'lucide-react'
+import { LayoutDashboard, Tags, FileText, Settings, LogOut } from 'lucide-react'
 import {
   Sidebar,
   SidebarContent,
@@ -30,6 +30,7 @@ export function AppLayout({ auth, children }: Props) {
   const navItems = [
     { path: '/', label: t('nav.dashboard'), icon: LayoutDashboard },
     { path: '/categories', label: t('nav.categories'), icon: Tags },
+    { path: '/template', label: t('nav.template'), icon: FileText },
     { path: '/settings', label: t('nav.settings'), icon: Settings },
   ]