docs: Add detailed R2 setup guide and clarify API token requirement

- Add new "Setup Guide" section to `docs/storage-r2.md` detailing the dual-credential requirement (S3 Keys + Cloudflare API Token) for full automation.
- Remove unsupported "manual/S3-only" workflow documentation to prevent user confusion.
- Update `js/services/r2Service.js` error message to explicitly prompt users for an API Token when the public domain is missing, instead of suggesting manual configuration.

js/services/r2Service.js

@@ -426,7 +426,7 @@ class R2Service {
 
       if (!publicBaseUrl) {
         throw new Error(
-          "No public bucket domain configured. Add an API token or configure the domain manually."
+          "No public bucket domain configured. Please add a Cloudflare API token to automate domain setup."
         );
       }