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.

install.sh

@@ -0,0 +1,51 @@
+#!/bin/bash
+set -e
+
+echo "Installing wgBill"
+echo "=================="
+
+# Backend
+echo ""
+echo "Setting up backend..."
+cd backend
+# python3 -> 3.14 on this machine has no prebuilt Pillow wheel yet and
+# fails building from source (missing jpeg headers); 3.12 has wheels.
+PYBIN=python3.12
+command -v "$PYBIN" >/dev/null 2>&1 || PYBIN=python3
+"$PYBIN" -m venv .venv
+source .venv/bin/activate
+python -m pip install -r requirements.txt
+
+if [ ! -f .env ]; then
+    echo "Creating backend .env from example..."
+    cp .env.example .env
+fi
+
+cd ..
+
+# Frontend
+echo ""
+echo "Setting up frontend..."
+cd frontend
+npm install
+
+if [ ! -f .env ]; then
+    echo "Creating frontend .env from example..."
+    cp .env.example .env
+fi
+
+cd ..
+
+echo ""
+echo "=================================="
+echo "Installation complete!"
+echo ""
+echo "Fill in backend/.env (Nextcloud app password, LLM API key) before"
+echo "first run - see backend/.env.example for what's needed."
+echo ""
+echo "To start the application:"
+echo "  ./run.sh"
+echo "or manually:"
+echo "  Backend:  cd backend && source .venv/bin/activate && python run.py"
+echo "  Frontend: cd frontend && npm run dev"
+echo ""