feat: polish simple login home

[yangshu2087]

Summary

• Restore / unauthenticated homepage to the user-approved simple login flow.
• Make X login the single high-priority CTA: 使用 X 登录，免费试用.
• Keep local/dev and V4 entries available as low-emphasis secondary actions.
• Add three concise capability cards: 风格学习, 推理生成, 发布执行.
• Update web unit and Playwright coverage plus handoff audit evidence.

Scope / API contract

• Frontend-only homepage polish in apps/web/components/v3/home-page.tsx.
• No public API contract changes.
• Existing calls remain startXOAuth() and createLocalSession().
• Existing error semantics are preserved through toUiError(...).
• No OAuth final authorization, real posting, or payment action is automated.
• Permissions and data consistency unchanged; no token/secret/storage shape changes.

Verification

• npm_config_cache=/tmp/draftorbit-npm-cache npx pnpm@10.23.0 --filter @draftorbit/web typecheck ✅
• npm_config_cache=/tmp/draftorbit-npm-cache npx pnpm@10.23.0 --filter @draftorbit/web test ✅ (32/32 node tests, 5/5 Playwright; harness 11.74s)
• NEXT_PUBLIC_API_URL=http://127.0.0.1:4311 npm_config_cache=/tmp/draftorbit-npm-cache npx pnpm@10.23.0 --filter @draftorbit/web build ✅ (14/14 pages)
• git diff --check ✅

Browser / visual evidence

• browser-use real in-app browser check on http://127.0.0.1:3400/ ✅
• Verified title, black X login CTA, trial copy, local/V4 secondary links, three cards, and zero console errors.
• Screenshot: output/playwright/manual-check/simple-login-home-2026-04-25.png (local ignored artifact; path referenced in docs/agent-handoff.md).
• Responsive checks: 375 / 768 / 1024 / 1440, no horizontal overflow, main CTA visible, cards visible.

Handoff

See docs/agent-handoff.md section: Simple login home polish (2026-04-25).