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/hooks/useTemplate.ts

@@ -3,7 +3,6 @@ import {
   template as templateApi,
   categories as categoriesApi,
   type TemplateDetail,
-  type TemplateItem,
   type Category,
   type ItemTier,
 } from '@/lib/api'