feat(web): load chat after Nuri wallet login

[eminogrande]

Summary

• replace the post-wallet repository empty state with a real browser Channel chat
• add NIP-42-authenticated publish and live subscription support to the web Nostr client
• discover/join an unambiguous open #general, load history, dedupe live events and send signed kind-9 messages
• preserve the repository browser at /repos and preserve the in-memory Nuri signer across SPA route changes

Security

• chat requires the unlocked Nuri or NIP-07 signer; it never falls back to an ephemeral read identity
• private key material remains tab-memory-only and existing lock zeroization is unchanged
• relay writes remain NIP-42 authenticated and channel-scoped by h tags

Verification

• cd web && pnpm check
• cd web && pnpm typecheck
• cd web && pnpm test:unit — 9 passed
• cd web && pnpm build
• cd web && pnpm test:e2e:smoke — 6 passed

Follow-ups

Multi-Space self-service, shared Categories and the chat.nuri.com / *.relay.nuri.com topology are tracked separately in #4, #5 and #6.

Closes #3