REST/HTTP gateway and web dashboard

## Description

Per spec Phase 11 (T127-T128), the project needs:

1. **REST/HTTP+JSON gateway**: generated from protobuf schema (grpc-gateway style) for CLI/GUI/third-party integration
2. **Web dashboard**: React SPA (static CDN-servable) with donor and submitter workflows at parity with CLI

## Requirements

- REST gateway exposes all gRPC services via HTTP+JSON
- Auto-generated from .proto files (tonic-web or custom gateway)
- Web dashboard: donor status, job submission, governance voting
- Static SPA deployable to CDN (no server-side rendering required)
- Authentication via Ed25519 signed tokens
- Rate limiting per contracts/README.md rate limit classes

## Success Criteria

- [ ] All 6 gRPC services accessible via REST/HTTP+JSON
- [ ] Web dashboard provides donor and submitter feature parity with CLI
- [ ] Dashboard deployable as static files
- [ ] Authentication and rate limiting enforced
- [ ] Integration tests for REST endpoints

## Testing (Principle V)

- Submit job via REST API → verify completion
- Fetch donor status via REST → verify matches CLI output
- Cast governance vote via web dashboard → verify recorded
- Rate limit testing: exceed configured limits → verify 429 response