fix: QA audit — lint cleanup, i18n fixes, module visibility, sidebar UX

- Fix 97 lint errors → 0 (unused imports, params, variables across 40+ files)
- Fix i18n key format: colon → dot notation for next-intl compatibility
- Add missing i18n keys (routes.application, routes.home, confirm)
- Fix module visibility: sidebar now respects per-account DB features
- Fix inject function: use dot-notation keys, add collapsed:true defaults
- Fix ConfirmDialog: use useTranslations instead of hardcoded German defaults
- Fix events page: replace placeholder 'Beschreibung' with proper description
- Fix Dockerfile: add NEXT_PUBLIC_CI ARG for Docker builds
- Collapse secondary sidebar sections by default for cleaner UX

apps/web/app/api/healthcheck/route.ts

@@ -26,7 +26,7 @@ async function getSupabaseHealthCheck() {
   try {
     const client = getSupabaseServerAdminClient();
 
-    const { data, error } = await client
+    const { data: _data, error } = await client
       .from('config')
       .select('billing_provider')
       .limit(1)