Add static GitHub Pages marketing site

[tomqwu]

Summary

• New single-page static marketing site under site/ (hero, feature grid, plan-limits table mirroring UsageService.PLAN_LIMITS, GitHub CTA). Brand-aligned with the app CSS, dark-mode aware, no external assets so it works offline on Pages.
• site/.nojekyll so Pages serves files as-is (no Jekyll).
• .github/workflows/pages.yml: deploys site/ to GitHub Pages via actions/deploy-pages on push to main touching site/**, plus manual workflow_dispatch.

This is a user-requested addition alongside the overnight e2e run — it does not change any application code or the e2e/CI lanes.

One-time setup to go live + point a domain over

1. Repo → Settings → Pages → Build and deployment → Source: GitHub Actions. (Required for the workflow to publish.)
2. Trigger the first deploy: merge this PR (push to main runs the workflow) or run the Deploy GitHub Pages workflow manually (workflow_dispatch).
3. Custom domain: Settings → Pages → Custom domain → enter your domain and Save. GitHub creates the CNAME for you (we intentionally did not commit a CNAME file). Then add DNS at your registrar:
   • Apex (example.com): A records → 185.199.108.153, 185.199.109.153, 185.199.110.153, 185.199.111.153 (and/or AAAA to the GitHub Pages IPv6 set).
   • Subdomain (www.example.com): CNAME → tomqwu.github.io.
4. Enable Enforce HTTPS once the cert provisions.

The CTA buttons point at the GitHub repo until a hosted app URL exists; swap the https://github.com/tomqwu/SignUpFlow links in site/index.html when that changes.

Test plan

• pages.yml is valid YAML
• CI-lane guard test still passes (tests/unit/test_e2e_lane.py)
• CI green (lint/type/test + e2e — unchanged by this PR)
• After merge: enable Pages (Source: GitHub Actions), confirm the site renders, set custom domain

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