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.

lib/constants.dart

@@ -0,0 +1,35 @@
+import 'package:flutter/material.dart';
+import 'package:google_fonts/google_fonts.dart';
+
+const int kInventoryId = 111062;
+
+TextStyle kLoginTitleStyle(Size size, {Color color = Colors.black}) => GoogleFonts.ubuntu(
+  fontSize: size.height * 0.060,
+  fontWeight: FontWeight.bold,
+  color: color
+);
+
+TextStyle kLoginSubtitleStyle(Size size) =>
+    GoogleFonts.ubuntu(fontSize: size.height * 0.030);
+
+TextStyle kLoginTermsAndPrivacyStyle(Size size) =>
+    GoogleFonts.ubuntu(fontSize: 15, color: Colors.grey, height: 1.5);
+
+TextStyle kHaveAnAccountStyle(Size size) =>
+    GoogleFonts.ubuntu(fontSize: size.height * 0.022, color: Colors.black);
+
+TextStyle kLoginOrSignUpTextStyle(Size size) => GoogleFonts.ubuntu(
+  fontSize: size.height * 0.022,
+  fontWeight: FontWeight.w500,
+  color: Colors.deepPurpleAccent,
+);
+
+TextStyle kTextFormFieldStyle({
+  Color color = Colors.black54,
+  fontSize,
+  fontWeight,
+}) => GoogleFonts.ubuntu(
+  color: color,
+  fontSize: fontSize,
+  fontWeight: fontWeight,
+);