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/App.tsx

@@ -7,6 +7,7 @@ import { RegisterPage } from '@/pages/RegisterPage'
 import { DashboardPage } from '@/pages/DashboardPage'
 import { CategoriesPage } from '@/pages/CategoriesPage'
 import { SettingsPage } from '@/pages/SettingsPage'
+import { TemplatePage } from '@/pages/TemplatePage'
 import '@/i18n'
 
 export default function App() {
@@ -35,6 +36,7 @@ export default function App() {
         <Routes>
           <Route path="/" element={<DashboardPage />} />
           <Route path="/categories" element={<CategoriesPage />} />
+          <Route path="/template" element={<TemplatePage />} />
           <Route path="/settings" element={<SettingsPage user={auth.user} onUpdate={auth.refetch} />} />
         </Routes>
       </AppLayout>