Initial commit: project scaffold with android and backend submodules

Set up monorepo structure with git submodules:
- android/ -> SnapLedger-Android (Kotlin + Jetpack Compose)
- backend/ -> SnapLedger-Backend (Python FastAPI)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitmodules

@@ -0,0 +1,6 @@
+[submodule "android"]
+	path = android
+	url = ../SnapLedger-Android
+[submodule "backend"]
+	path = backend
+	url = ../SnapLedger-Backend