seo: add SiteNavigationElement structured data (Editor first)

Google's auto-generated sitelinks for "velxio" surfaced docs/blog pages
but not the Editor (the primary app) or Home. Sitelinks can't be forced,
but an explicit SiteNavigationElement ItemList naming the primary nav —
Editor, Examples, Documentation, Pricing, About, in that order — is the
recognized structured-data hint for what the site's main sections are.

frontend/index.html

@@ -194,6 +194,50 @@
             "@type": "ReadAction",
             "target": "https://velxio.dev/"
           }
+        },
+        {
+          "@type": "ItemList",
+          "@id": "https://velxio.dev/#primary-nav",
+          "name": "Velxio primary navigation",
+          "description": "The main sections of Velxio — the simulator Editor is the primary entry point.",
+          "itemListOrder": "https://schema.org/ItemListOrderAscending",
+          "itemListElement": [
+            {
+              "@type": "SiteNavigationElement",
+              "position": 1,
+              "name": "Editor",
+              "description": "Open the free multi-board simulator editor — write, compile and run Arduino, ESP32, RP2040 and RISC-V code in your browser.",
+              "url": "https://velxio.dev/editor"
+            },
+            {
+              "@type": "SiteNavigationElement",
+              "position": 2,
+              "name": "Examples",
+              "description": "100+ ready-to-run Arduino, ESP32 and analog SPICE example projects.",
+              "url": "https://velxio.dev/examples"
+            },
+            {
+              "@type": "SiteNavigationElement",
+              "position": 3,
+              "name": "Documentation",
+              "description": "Guides for the editor, ESP32/RISC-V emulation, custom chips and self-hosting.",
+              "url": "https://velxio.dev/docs"
+            },
+            {
+              "@type": "SiteNavigationElement",
+              "position": 4,
+              "name": "Pricing",
+              "description": "Free forever for the simulator; paid plans raise the AI-agent quota.",
+              "url": "https://velxio.dev/pricing"
+            },
+            {
+              "@type": "SiteNavigationElement",
+              "position": 5,
+              "name": "About",
+              "description": "About Velxio, the free open-source multi-board embedded simulator.",
+              "url": "https://velxio.dev/about"
+            }
+          ]
         }
       ]
     }