feat(landing): add Atlas landing page with Astro static site

Implement the full landing page for atlas.atomstorm.ai per the PRD at
Docs/Execution/Landing-Page-PRD-2026-03-14.md. Includes design spec,
bilingual Astro site, CI pipeline, and all assets.

Key deliverables:
- DESIGN.md: 7-section Atom Web Design specification
- Astro 5.x static site with 15 components and 11 page sections
- Bilingual i18n (zh/en) with path-based routing
- Build-time GitHub release manifest integration
- Release channel state machine (Stable/Prerelease/Coming Soon)
- CSS design tokens mapped from AtlasBrand.swift
- Self-hosted fonts (Space Grotesk, Instrument Sans, IBM Plex Mono)
- OG social sharing images for both locales
- GitHub Actions workflow for GitHub Pages deployment
- Zero client JS, 227KB page weight, 17/17 quality checks passed

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Docs/Backlog.md

@@ -51,6 +51,7 @@
 - `EPIC-18` Public Beta Feedback and Trust Closure
 - `EPIC-19` GA Recovery and Execution Hardening
 - `EPIC-20` GA Launch Readiness
+- `EPIC-21` Marketing Site and Direct Distribution Landing Page
 
 ## Now / Next / Later
 
@@ -282,6 +283,15 @@
 - `ATL-235` Run a trusted hardware-diverse signed beta cohort — `Product Agent`
 - `ATL-236` Triage public-beta issues before any GA candidate naming — `Product Agent`
 
+#### Launch Surface Phase 5: Landing Page and Domain
+
+- `ATL-241` Finalize landing-page PRD, CTA policy, and bilingual information architecture — `Product Agent`
+- `ATL-242` Design and implement the marketing site in `Apps/LandingSite/` — `Mac App Agent`
+- `ATL-243` Add GitHub Pages deployment workflow and environment protection — `Release Agent`
+- `ATL-244` Bind and verify a dedicated custom domain with HTTPS enforcement — `Release Agent`
+- `ATL-245` Surface release-channel state, download guidance, and prerelease install help on the page — `UX Agent`
+- `ATL-246` Add privacy-respecting analytics and launch QA for desktop/mobile — `QA Agent`
+
 ## Definition of Ready
 
 - Scope is clear and bounded