Import documentation: cshare-app/README.md

Import the existing local project snapshot. This commit organizes existing files; it does not represent newly developed functionality.

cshare-app/README.md

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+# CShare Frontend
+
+React/Vite frontend for CShare.
+
+## Commands
+
+```sh
+npm install
+npm run dev
+npm run check
+npm run build
+npm run preview
+```
+
+`npm run build` emits production assets to `dist/` and precompresses them with gzip and Brotli.
+
+## Configuration
+
+Set `VITE_API_BASE_URL` when the API is hosted on a different origin:
+
+```sh
+VITE_API_BASE_URL=https://api.example.com
+```
+
+If omitted in production, the app uses same-origin API calls, which is the preferred setup behind an Nginx reverse proxy.