Frontend still targets retired conduit.productionready.io API host

### Summary
The AngularJS frontend is still hard-coded to call `https://conduit.productionready.io/api`. That host now returns a 307 redirect to `https://api.realworld.io/api`, which then fails with Cloudflare DNS errors, so all API calls from the app end up blocked by CORS or return 5xx responses.

### Steps to Reproduce
1. Clone the repo and install dependencies (`npm install`).
2. Run `gulp` (Node 10.24.1 via nvm).
3. Open `http://localhost:4000` and watch the network console.

### Current Behavior
- Requests for `/articles` and `/tags` redirect to the defunct `api.realworld.io` origin and fail with `Access-Control-Allow-Origin` missing, yielding `status: -1` in Angular.
- The home feed stays in the loading state and login/logout flows are unreliable because the API never returns data.
- The failing responses bubble up as `Possibly unhandled rejection` errors in the console. When the redirect does succeed, the backend responds with `offset=NaN`, leading to a 500 "Internal Server Error".

### Expected Behavior
The frontend should point to the active RealWorld demo API so data loads normally and authentication continues to work.

### Proposed Fix
- Update `src/js/config/app.constants.js` to use `https://api.realworld.show/api`, which is the backend referenced in the main RealWorld README.
- Refresh the README section that mentions the live API host so new users start from the correct URL.
- Guard the pagination component so it always computes a numeric `limit`/`offset` before issuing requests; this prevents `offset=NaN` queries that the new backend rejects with 500s.

With those changes applied locally, the home page loads articles and tags without CORS errors, and other core flows (login, favorites, editing) work again.