feat: Unleash feature flags via src/lib/flags.tsx, hello-banner on home page

Ported from service-template-react e6f8827.

README.md

@@ -21,13 +21,35 @@ npm run format
 image runs in every environment. Read values through `getConfig()` in
 `src/lib/config.ts`.
 
-| Env var        | Default | Meaning                                         |
-| -------------- | ------- | ----------------------------------------------- |
-| `API_BASE_URL` | `/api`  | Backend base URL; `/api` is the same-host route |
-| `APP_NAME`     | `app`   | Display name                                    |
+| Env var                  | Default | Meaning                                             |
+| ------------------------ | ------- | --------------------------------------------------- |
+| `API_BASE_URL`           | `/api`  | Backend base URL; `/api` is the same-host route     |
+| `APP_NAME`               | `app`   | Display name                                        |
+| `UNLEASH_URL`            | (empty) | Unleash base URL for the browser; empty = flags off |
+| `UNLEASH_FRONTEND_TOKEN` | (empty) | Unleash frontend token (public by design)           |
 
 Everything in `config.js` is public. Never put secrets there.
 
+## Feature flags
+
+Flags come from Unleash (UI: http://unleash.192.168.1.83.nip.io, environment `development`)
+through its Frontend API, using the official `@unleash/proxy-client-react` SDK.
+`src/lib/flags.tsx` wraps it: `FlagsProvider` is mounted once in `main.tsx`;
+components call the wrappers, never the SDK directly:
+
+```tsx
+import { useFlag, useVariant } from "../lib/flags";
+const showBanner = useFlag("hello-banner");
+```
+
+- In the cluster the deployment sets `UNLEASH_URL=http://unleash.192.168.1.83.nip.io`
+  and `UNLEASH_FRONTEND_TOKEN` from Secret `unleash-client-token` (key `frontendToken`).
+- The SDK fetches on load and refreshes every 15 s; components re-render on change.
+  A reload always picks up the current state.
+- With `UNLEASH_URL` unset (local `npm run dev`, tests) every flag reads `false`, nothing is fetched.
+- Tests: `vi.mock("@unleash/proxy-client-react")` and control `useFlag` (see `src/app.test.tsx`).
+- `hello-banner` on the home page is the platform demo flag; delete it when you don't need it.
+
 ## Container
 
 - `GIT_SHA` build arg is baked in as `import.meta.env.VITE_GIT_SHA`.