docs: refresh package READMEs, npm metadata, and repo social preview

[pietechsolution]

Documentation and package-metadata pass over the three published wrappers. No
source, build, or release behaviour changes.

What changed

• packages/react/README.md — quick start for both SeatingChart and
  SeatPicker, security boundary, architecture note, complete callback-prop and
  imperative-handle lists, FAQ, and the shared SDK ecosystem table.
• packages/vue/README.md, packages/angular/README.md — same structure, plus
  the chart inputs, outputs, and handle methods that the tables were missing
  (initialView, errorDisplay, buyerAccessTokenProvider,
  buyerAccessToken, the three buyer-access events, and refreshAccess).
• README.md — rewritten around the three published packages, with the
  security and public/private boundaries, an FAQ, and the ecosystem table.
• packages/*/package.json — description and keywords aligned across the
  three packages.
• .github/social-preview.png — 1280x640 repository social card.

Verification

Every documented prop, event, and method was read out of the source before it
was written down:

• handle methods and identity props: SEATING_CHART_HANDLE_METHODS /
  SEATING_CHART_IDENTITY_PROPS in @seatlayer/js
• Vue props/emits: packages/vue/src/SeatingChart.ts
• Angular inputs/outputs: packages/angular/src/seating-chart.component.ts
• React props/handles: packages/react/src/{SeatingChart,SeatPicker}.tsx
• shipped type declarations and entry points: each package.json exports

Every non-npm link in the changed files returns HTTP 200; npm package pages
return 403 to curl (bot protection).

No version bump and no release: the README changes reach npm with the next
scheduled wrapper release.

The social preview cannot be set through the API — it needs to be uploaded once
under Settings -> General -> Social preview.