docs: rewrite README to reflect current state

[rabble]

Summary

Rewrites README.md to accurately reflect the current @divinevideo/login API and behavior, verified against src/ and tests/.

Verified changes

• Corrected getAuthorizationUrl() return — documents { url, pkce } (previously implied { url }), and explains when the verifier must be passed to exchangeCode.
• Corrected Quick Start flow — uses parseCallback() and handles the createRpc() null case (no access_token), matching the real factory return.
• Documented the onUnauthorized auto-refresh hook on DivineRpc, wired to getSessionWithRefresh(), with a real usage snippet.
• Documented resilience behavior now in the code: 429 retry with exponential backoff + Retry-After, 30s request timeouts, and cross-tab refresh serialization via the Web Locks API.
• Completed the API reference — added refreshSession, getAuthorizationHandle, isExpired, toStoredCredentials, DivineRpc.fromServerUrl, RpcError, and the exported generatePkce/validatePkce utilities and public types.
• Added a Browser (CDN) section — DivineLogin global from unpkg/jsDelivr and the @divinevideo/login/browser export.
• Added a Development section — build/dev/lint/test scripts from package.json; CI runs lint, test, and build on Node 22.
• Clarified configuration — policy:full default scope, divine_* storage keys, optional nostr-tools peer dependency for BYOK, and TokenResponse fields.

Kept the existing brand-aligned framing, the keycast-login migration section, and the standard Divine footer.

Test plan

Docs-only change. Verified every documented symbol, signature, default, and endpoint against src/index.ts, src/oauth.ts, src/rpc.ts, src/types.ts, src/pkce.ts, package.json, and .github/workflows/ci.yml.

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