Add full app source

Adds the Flutter inventory app source, cleaned up for a shared repo:
generic package name, no hardcoded ERP endpoints (moved to gitignored
local config), no dead auth code, no debug logging of session data.

.fvm/fvm_config.json

@@ -0,0 +1,4 @@
+{
+  "flutterSdkVersion": "3.29.2",
+  "flavors": {}
+}