feat: secure Stellar signing, cross-tab auth, live feed, SBTi dashboard - #609

Summary

closes #542 — Secure Stellar key management
Backend corporate-platform-backend/src/stellar/signing/

• SigningProvider interface: getPublicKey(), signTransaction(), isLive()
• EnvSigningProvider (default/dev) + KmsSigningProvider (prod skeleton)
• Distinct providers: SIGNING_PROVIDER_CONTRACT / SIGNING_PROVIDER_TRANSFER
• SorobanService + TransferService inject providers — no process.env.STELLAR_SECRET_KEY outside signing/
• Explicit STELLAR_SIGNING_MODE=simulate|live (no silent missing-key simulate)
• Startup fail-fast in live mode when secret/public key invalid
• Audit logs include public key + keyId per signature
• config.schema.ts validates signing env vars
• KEY_ROTATION.md + unit tests for misconfig / simulate mode

closes #550 — Cross-tab auth sync
Web AuthContext + lib/auth/cross-tab-auth.ts

• BroadcastChannel('cs-auth') for login / logout / refresh
• storage listener fallback (older browsers)
• Logout clears all tabs; login/refresh re-runs syncProfile
• Leader election (cs_auth_leader) so only one tab proactive-refreshes
• Unit tests for storage keys, leader lock, broadcast safety

closes #552 — Live retirement feed reconnect
Web components/feed/LiveRetirementFeed.tsx

• Real WebSocket to NEXT_PUBLIC_RETIREMENT_STREAM_URL (or /api/v1/retirements/stream)
• Exponential backoff reconnect (cap 30s)
• States: connecting / connected / reconnecting / disconnected / error + Retry
• Pause closes the socket; visibility pause when tab hidden
• Dedup by event id; skeleton while connecting; no mock companies
• Empty components/retirement/LiveRetirementFeed.tsx → re-export

closes #546 — SBTi dashboard aggregation
Backend ProgressTrackingService + SbtiService.getDashboard()

• Completion %, linear on-track/behind/ahead, time-series per target
• Status summary counts + scope 1/2/3 rollups
• Reuses getRetirementGap(); failures logged → dataQuality: 'partial'
• 3-minute in-memory cache
• Unit tests for on-track / behind / ahead / missing data

Verify

# Backend
cd corporate-platform/corporate-platform-backend
npm test -- signing
npm test -- progress-tracking

# Web
cd corporate-platform/corporate-platform-web
npm test -- cross-tab-auth
npm test -- LiveRetirementFeed

Closes #542
Closes #550
Closes #552
Closes #546