Add README

Logo + name header matching the ~/Projects/gain style, quick start
(native + Docker), tech stack, how-it-works walkthrough, config, and
project structure.

README.md

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+<h1 align="center">
+  <img src='./frontend/public/favicon.svg' width="150px" style="margin-top: 20px;">
+  <br>
+  <b>wgBill</b>
+  <br>
+</h1>
+
+Photograph a shared grocery receipt, pick who's splitting what, and it uploads
+the photo to Nextcloud, shares it, and creates the matching bill(s) in
+Cospend automatically.
+
+## Quick Start
+
+```bash
+# Install dependencies
+./install.sh
+
+# Start both backend and frontend in tmux
+./run.sh
+```
+
+The app will be available at `http://localhost:5173` (frontend) and
+`http://localhost:5000` (backend).
+
+### Docker
+
+```bash
+cp backend/.env.example backend/.env   # fill in, see below
+docker compose up -d --build
+```
+
+Available at `http://localhost:8080` (override with `FRONTEND_PORT`). Put
+your own TLS-terminating reverse proxy in front for anything beyond
+localhost, and set `SESSION_COOKIE_SECURE=true` once you do.
+
+## Tech Stack
+
+- **Frontend:** React, Vite, TypeScript, Tailwind CSS, installable PWA
+- **Backend:** Flask, SQLite
+- **Auth:** Nextcloud Login Flow v2 - each user logs in with their own NC
+  account, no shared credentials
+- **Receipt extraction:** any OpenAI-compatible vision model (OpenAI,
+  Gemini, ...), configurable, not hardcoded to one provider
+
+## How it works
+
+1. Take a photo of a receipt (or drop one in on desktop).
+2. A vision model extracts the store name, date, and line items - fix
+   anything it got wrong.
+3. Tap items to include them (splits with everyone by default), or expand
+   one to pick specific people instead.
+4. The app groups items by identical split and creates one bill per group -
+   a Cospend bill can only have one split, so a receipt with a "shared" item
+   and a "just me" item becomes two bills automatically.
+5. Each bill's photo is uploaded to the submitting user's own Nextcloud
+   files, shared as a public link, and the bill is created in Cospend with
+   that link in its title (Cospend's UI only picks up a link there, not in
+   the comment) and an itemized breakdown in the comment.
+
+## Configuration
+
+Copy `backend/.env.example` to `backend/.env` and fill in:
+- Your Nextcloud server address
+- A vision provider (base URL, API key, model) - OpenAI, Gemini's
+  OpenAI-compatible endpoint, or anything else speaking that API
+- `TOKEN_ENCRYPTION_KEY` (generate with the command in the file) - encrypts
+  stored Nextcloud app passwords at rest
+
+There's nothing to configure for auth beyond that - users log in themselves
+via Nextcloud, nothing is hardcoded per-account.
+
+## Project Structure
+
+```
+backend/
+  app/
+    auth.py, auth_routes.py   # Nextcloud Login Flow v2, sessions
+    nc_client.py               # WebDAV upload + public share (per user)
+    cospend_client.py          # Cospend OCS API (per user)
+    llm_client.py               # receipt -> {store, date, items} extraction
+    routes.py                   # /api/* - receipts, groups, bills
+  schema.sql                    # users, sessions, receipts, groups
+frontend/
+  src/
+    components/
+      LoginGate.tsx              # login flow UI
+      CaptureStep.tsx            # photo capture / drag-drop
+      GroupStep.tsx               # item review + splitting + submission
+      SummaryStep.tsx
+    api.ts                       # backend client
+```
+
+## Documentation
+
+- **[PLAN.md](PLAN.md)** - original feasibility research and architecture
+  plan (Nextcloud/Cospend API details, verified against real source)