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/package.json

@@ -0,0 +1,26 @@
+{
+  "name": "frontend",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "lint": "oxlint",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "react": "^19.2.8",
+    "react-dom": "^19.2.8",
+    "vite-plugin-pwa": "^1.3.0"
+  },
+  "devDependencies": {
+    "@types/node": "^24.13.3",
+    "@types/react": "^19.2.18",
+    "@types/react-dom": "^19.2.4",
+    "@vitejs/plugin-react": "^6.1.0",
+    "oxlint": "^1.79.0",
+    "typescript": "~6.0.2",
+    "vite": "^8.2.2"
+  }
+}