Initial scaffold: Flask/SQLite backend, React/Vite PWA frontend

Backend: receipt upload -> LLM vision extraction (OpenAI-compatible,
provider-agnostic), item review/edit, per-group splitting against
Cospend projects/members, highlight+upload via WebDAV, public share
link, bill creation via Cospend's OCS API (verified against real
source, not just doc summaries).

Frontend: capture -> review -> group -> summary flow as an installable
PWA.

install.sh / run.sh (venv + npm, tmux session) instead of Docker, per
the ~/Projects/gain pattern.

frontend/.env.example

@@ -0,0 +1,4 @@
+# Leave empty for local dev (vite.config.ts proxies /api -> localhost:5000).
+# Set to the deployed backend's origin when building for production, e.g.
+# VITE_API_BASE_URL=https://wgbill-api.example.com
+VITE_API_BASE_URL=