feat(ui): guest browse mode — browse marketplace without login (US-1)

[argszero]

Summary

Implements guest browse mode (US-1) — the UI prototype previously required login to view the marketplace, but the user story requires that visitors (not logged in) can browse the model market first.

Host rant: 2026-08-15T08:57:37 — "UI 原型补齐"访客未登录浏览市场"功能（US-1）"

Changes

• Login page: added a "先逛逛市场 (Guest browse)" entry button, plus a points mechanism explanation (US-2) block (1 pt ≈ 1 CNY, fractional consumption, daily gift 1 pt, 10% platform fee)
• Guest mode: entering as guest opens the Marketplace directly — guest can browse, search, filter by provider, sort, and see point prices
• Guest restrictions:
  • "使用 / 消费" (use/consume) prompts "请先登录" and does not open the chat mock
  • Sidebar shows only "模型市场" (mode label: 游客模式 · 仅浏览市场); wallet / sharing / transactions / settings / admin views are blocked with a login prompt
• State transitions: login exits guest mode and restores the full nav; logout returns to the login page (guest entry visible again)
• ui/README.md documents the guest browse mode

Related Issue

No issue (rant-driven; issue #6 remains unclaimed)

Tests

• cargo test passes (1 passed)
• cargo fmt --check passes
• node --check ui/js/app.js passes
• New unit tests added (N/A — UI prototype, no test harness)

Checklist

• Branch naming follows convention (feat/ui-guest-browse)
• Conventional Commits message
• Single purpose, minimal change