feat: headless useESignature hook and ESignature theming for RN and web

[grimen]

Why

The ESignature component hardcoded every color and string (iOS-blue buttons, "Review and sign your document", "Cancel", ...) and the only customization was label. There was also no way to start the signing flow from a host's own UI: the state machine (offline check, source.start(), WebView/iframe embed, session-expiry restart, success delay, error mapping) was welded to the built-in screens. Esaú hit the first problem ("it would be great if the library allowed changing the button colors"); the second is the underlying design issue: the library should own the protocol and the state machine, not the chrome.

What

Headless hook, both packages. The state machine moves into useESignature(options), exported from @blinkbitcoin/esign-react-native (and its Apollo-free /webform subpath) and @blinkbitcoin/esign-react. It returns status, error, isSessionExpired, the actions sign / cancel / retry / restart / checkConnection, and an embed handle:

• RN: webViewProps to spread onto the host's own WebView (plus isCheckingConnection).
• Web: embed, either { kind: 'iframe', iframeProps } or { kind: 'mount', ref } for DocuSign.js sources. The mount effect now waits for the container to attach via a callback ref instead of assuming it exists on the first signing render.

Thin default UI with theming. ESignature is now a consumer of the hook and gains three optional props: theme (primaryColor, primaryTextColor, mutedTextColor, successColor, errorColor, warningColor), styles (per-element overrides), and labels (copy overrides; title/sign default to label). Precedence is base < theme < styles. Esaú's ask becomes theme={{ primaryColor: '#F7931A' }}.

No breaking change. Existing props keep working and the default look and copy are byte-identical. All testIDs are unchanged, so Maestro and Playwright flows are untouched.

Demos. Both demo apps gain a HookSigning screen (host-owned buttons + WebView/iframe over the hook) behind a toolbar toggle; the default UI stays the initial view and the E2E target.

Docs. READMEs gain Theming and Headless usage sections; architecture pages, source tree, integration notes, CLAUDE.md/AGENTS.md, and the component-hierarchy diagram reflect the split.

Tests

• RN package: 74 Jest tests (was 36), web package: 89 (was 43), both at the enforced 100% coverage. State-machine tests moved to useESignature.test.tsx, component tests cover screens + wiring + theming, theme.test.tsx covers the resolvers, and each package has a hook-only consumer test that never imports ESignature.
• Demos: RN 22 tests, web 14, above their coverage floors.
• npm run typecheck, npm run lint, npm test (all workspaces), npm run build && npm run check:packages, make docs-check all green locally.

[grimen]

Demo of the themed path on the iOS simulator (mock provider): stock component in the default blue → toolbar switch to the themed mode → same component with Blink's palette (#fc5805, pill buttons) and the label Sign the agreement → signing page in the WebView → success → host alert.

The demo app's toolbar now cycles default → themed → hook (dfa3cb6); themed is three props on the stock component:

<ESignature
  source={source}
  label="Sign the agreement"
  theme={{ primaryColor: '#fc5805', primaryTextColor: '#FFFFFF', mutedTextColor: '#9292A0', successColor: '#00A700', warningColor: '#ffad0d' }}
  styles={{ button: { borderRadius: 50, paddingHorizontal: 32 } }}
/>

GIF lives on the assets/pr-60-demo branch (delete after merge).