Implement wallet journey state, UI marker, tests, and regtest/dev docs

[Bluuefanatic]

Summary

This competency test PR adds a deterministic wallet journey state model in the frontend, exposes it in the main wallet view for UI-state verification, adds RTL coverage, and documents both friction findings and practical regtest simulation steps.

What Changed

• Added typed wallet journey state hook:
  • useWalletJourneyState.ts
  • Returns a single state string with deterministic logic and no side effects.
  • Uses existing wallet and service/session values only (no backend changes).
• Integrated journey state into main wallet page:
  • MainWalletPage.tsx
  • Added data-journey-state on the top-level container.
• Added React Testing Library coverage for journey state attribute:
  • MainWalletPage.test.tsx
  • Mocks wallet and service/session inputs.
  • Verifies at least two journey states:
    • no_wallet_or_not_initialized
    • coinjoin_in_progress
• Added one-page wallet friction audit:
  • wallet-state-friction-audit.md
  • Covers silent stuck points and practical UX improvements across key routes.
• Added concise developer docs for regtest state simulation:
  • developing.md
  • New section: Simulating Wallet Journey States (Regtest)
  • Includes steps for empty wallet, funded wallet, coinjoin-ready state, UI verification, and contributor tips.

Why

• Establishes a clear, reusable state model for wallet journey UX.
• Makes state transitions observable in the DOM for testing and debugging.
• Improves confidence with focused RTL coverage.
• Documents route-level friction and practical contributor workflows for faster iteration.

Validation

• Ran: npm test -- MainWalletPage.test.tsx
• Result: passing

Scope Notes

• Frontend-only changes.
• No API or backend contract changes.
• No behavior/layout changes beyond adding the data-journey-state attribute and docs/tests additions.