The Instant Digital Wallet for Novels, Micro-Unlocks & Tips

Act as a Senior Mobile and Web Developer. I need to integrate our standalone wallet service, SikkaPay, into our client application using the official SikkaPay REST API.

Here are the pre-configured credentials and endpoints for our registered app:
- App Name: NovelsVault Android App
- Client App ID: app_nv_android_908234
- Public API Key: sp_live_pk_8f93a201b947c6e2
- SikkaPay Base URL: https://sikkapay.kohnovelsurdu.com/api/v1
- Webhook Signature Secret: whsec_77a01b294c6f93e18a02

Implement the following flows cleanly with robust error handling:

1. User Authentication & Wallet Binding:
   - Check if the user has bound their SikkaPay account.
   - If not bound, redirect them to `https://sikkapay.kohnovelsurdu.com/oauth/authorize?client_id=app_nv_android_908234&scope=wallet:read,wallet:debit&redirect_uri=https://novelsvault.com/callback`.
   - Handle the callback token securely and store the authorized `sikkapay_access_token` in encrypted storage.

2. Balance Check & Unlock Action:
   - Make an authenticated GET request to `/v1/wallet/balance` with the user's bearer token.
   - Display the user's available Sikka credits inside our UI using clean, modern cards and responsive charts.
   - Before allowing a user to read a locked chapter, send a POST request to `/v1/content/unlock`:
     {
       "content_id": "CHAPTER_14_DIL_E_MUZTAR",
       "cost_sikka": 10,
       "author_id": "01m27ehaz7kgh5wevr7my36x8y"
     }
   - Verify the 200 OK response containing the verified transaction token before decrypting and presenting the novel text.

3. Webhook Receiver & OCR Proof Handler:
   - Write a secure endpoint `/api/sikkapay/webhook` that validates incoming HMAC-SHA256 signatures using our secret `whsec_77a01b294c6f93e18a02` from the `X-Sikka-Signature` header to process instant balance top-ups verified via picture OCR and payment email matching.