refactor : rename Inventory_frontend to frontend everywhere

Update Dockerfile, nginx, .dockerignore, makefile, CI workflow,
CLAUDE.md, README, release script to use frontend/ instead of
Inventory_frontend/. Remove submodule references from CI and
release script.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.dockerignore

@@ -6,9 +6,9 @@ docker/
 deploy/docker/docker-compose.prod.yml
 deploy/docker/deploy.sh
 deploy/docker/.env.example
-Inventory_frontend/node_modules
-Inventory_frontend/.nuxt
-Inventory_frontend/.output
+frontend/node_modules
+frontend/.nuxt
+frontend/.output
 var/
 vendor/
 LOG/
@@ -21,4 +21,4 @@ scripts/
 *.md
 !composer.lock
 !symfony.lock
-!Inventory_frontend/package-lock.json
+!frontend/package-lock.json