refactor : merge Inventory_frontend submodule into frontend/ directory

Merges the full git history of Inventory_frontend into the monorepo
under frontend/. Removes the submodule in favor of a unified repo.

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

.gitignore

@@ -34,10 +34,6 @@ FEATURE_IDEAS.md
 bin/.phpunit.result.cache
 ###< temp files ###
 
-###> frontend ###
-/frontend/
-###< frontend ###
-
 ###> ide ###
 /.idea/
 ###< ide ###