feat: BackendStatusBanner + backend API contract docs

[pharuq411]

Closes #12

Two deliverables:

BackendStatusBanner component — a dismissible red banner that appears on every page when the Octraban indexer API is unreachable.

docs/api.md — a complete, human-readable contract for every HTTP/SSE endpoint the frontend consumes.

Changes
src/components/BackendStatusBanner.tsx (new)
Polls GET /api/health on mount and every 30 s using AbortSignal.timeout(4000).

Renders a role="alert" banner only when the backend is down and the user hasn't dismissed it.

Shows the configured VITE_INDEXER_URL and links to the backend repo.

No external dependencies — plain fetch + React state.

src/App.tsx
Imports and mounts above the block so it appears on every route.

docs/api.md (new)
Documents all 30+ endpoints consumed by src/api.ts, grouped by resource.

Each entry covers: method + path, query params, request body (where applicable), and a typed response example.

Covers SSE streams (/sub-invocations/stream, /transactions/:txHash/status/stream).

Includes a standard error-response section and a quick-reference SSE table.

Testing
With the backend stopped: the banner appears within ~4 s of page load.

Clicking × dismisses it for the session.

With the backend running (npm run dev in octraban_backend): banner never appears.

npm run build passes — no new type errors.

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