feat: Product landing page at / with responsive design

- landing.html: Full product website (hero, features, how-it-works, platform coverage, pricing, FAQ, CTA, footer)
- main.py: Route / to landing.html (overrides StaticFiles default index.html)
- Workspace dashboard remains at /index.html
- Responsive: desktop nav, hamburger mobile menu, single-column on small screens
- No Vue/Element Plus dependency; standalone HTML+CSS+JS (~30KB)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

platform/backend/app/main.py

@@ -115,9 +115,20 @@ if AUTOMATION_IMAGES_DIR.exists():
 
 # 挂载前端
 FRONTEND_DIR = Path(__file__).parent.parent.parent / "frontend"
-if FRONTEND_DIR.exists() and (FRONTEND_DIR / "index.html").exists():
+if FRONTEND_DIR.exists() and (FRONTEND_DIR / "landing.html").exists():
+
+    # Landing page at / (overrides StaticFiles default index.html)
+    @app.get("/")
+    async def landing_page():
+        return FileResponse(str(FRONTEND_DIR / "landing.html"), media_type="text/html")
+
+    @app.get("/landing.html")
+    async def landing_direct():
+        return FileResponse(str(FRONTEND_DIR / "landing.html"), media_type="text/html")
+
+    # Workspace dashboard at /index.html (existing management UI)
     app.mount("/", StaticFiles(directory=str(FRONTEND_DIR), html=True), name="frontend")
-    logger.info(f"Frontend mounted at / from {FRONTEND_DIR}")
+    logger.info(f"Frontend mounted at / from {FRONTEND_DIR} (landing at /)")
 
     # SW 特殊处理
     sw_path = FRONTEND_DIR / "sw.js"