Refactor: split useNostr god-hook into focused hooks

[satsdisco]

Summary

useNostr.ts is 668 lines doing everything — auth, publishing, article fetching, relay management, zap verification. This makes it hard to reason about, test, and maintain.

Proposed Split

• useAuth.ts — Login/logout, NIP-07/NIP-46/nsec/Android signer, session management
• usePublish.ts — Article building, signing, relay publishing, zap-gated publishing
• useArticles.ts — Fetching articles, drafts, profiles from relays
• useRelays.ts — Relay list management, read/write relay preferences
• Keep useNostr.ts as a thin orchestrator that composes the above

Principles

• Each hook should be independently testable
• Shared state (pubkey, signer) passed via context or composition
• No circular dependencies between hooks

Acceptance Criteria

• Each new hook is <200 lines
• useNostr is a thin composition layer
• All existing functionality preserved
• ESLint passes
• Build compiles cleanly