feat(site): add official product landing page

apps/site/design-qa.md

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+# Design QA
+
+## Comparison target
+
+- Source visual truth: `C:\Users\david\.codex\generated_images\019f92c1-c086-7b62-9e3e-8942418c08bb\exec-92a2158c-b865-4c30-a1af-bd75a8410bd2.png` (the selected final design, option 1).
+- Supporting product references: `docs/screen/login.png`, `docs/screen/workspace.png`, `docs/screen/export.png`, `docs/screen/p3.png`, and `docs/screen/p4.png`.
+- Intended implementation: `apps/site/index.html` at a desktop viewport matching the selected long-form landing page.
+- Implementation screenshot: unavailable.
+
+## Evidence and state
+
+- Source assets were opened and inspected before implementation.
+- The page uses the supplied official logo and product screenshots rather than recreated logo or product-preview artwork.
+- Primary interactions in code: “下载” scrolls to the download section; “WEB 登录” links to `https://agent.senlin.ai`.
+- Static resource validation passed: all six referenced assets exist and the WEB login URL is present.
+- Backend tests and the existing `web_v1` production build passed.
+
+## Required fidelity surfaces
+
+- Fonts and typography: implemented with the existing Chinese system-font stack, a high-contrast display hierarchy, and compact utility text aligned with the source system.
+- Spacing and layout rhythm: implemented as an 1440px-centered desktop grid with a left text / right product-preview hero and responsive single-column fallback.
+- Colors and visual tokens: uses the source product’s cobalt `#165DFF`, pale gray workspace surfaces, thin blue-gray separators, and navy footer/status treatment.
+- Image quality and asset fidelity: uses supplied SVG logo and supplied product captures for workbench, exploration, documents, and AI conversation.
+- Copy and content: includes project context, exploration data, project documents, expert AI conversations, private deployment, and human confirmation.
+
+## Findings
+
+- [P1] Browser-rendered visual comparison is unavailable.
+  - Evidence: the in-app browser could not initialize in this environment because of a system path error, so no implementation screenshot or same-viewport composite comparison could be captured.
+  - Impact: typography, crop, spacing and responsive behavior have not been verified against the selected visual target in a browser.
+  - Fix: restore in-app browser access, capture the homepage at the selected desktop viewport, compare it side-by-side with the source visual, and resolve any P0/P1/P2 differences.
+
+## Comparison history
+
+- Initial pass: blocked before browser capture; no visual comparison iteration is available.
+
+## Final result
+
+final result: blocked