feat(web): add downloads page and wire account download card

The funnel dead-ended at "we'll email you" — there was no downloads
surface. Add a real /downloads page in the landing template style
(shared nav + footer + page shell) with client-side navigator.platform
OS detection. macOS (Apple Silicon .dmg) is featured as the shipping
build; Windows (.msi) and Linux (.deb/.AppImage) are linked but clearly
marked "coming soon" to match current product state — Windows chat and
the Linux soul backend are not yet ready, so presenting them as fully
working would overpromise.

Also un-break the account dashboard download card, which was hardcoded
disabled ("Shipping within 30 days"). It now links the live macOS
installer plus the full /downloads page.

src/main.el

@@ -87,6 +87,7 @@ from checkout        import { checkout_page }
 from safety          import { safety }
 from gallery         import { gallery_page }
 from account         import { account_page }
+from downloads       import { downloads_page }
 
 // ── Share-card HTML allowlist ─────────────────────────────────────────────────
 //
@@ -1971,6 +1972,15 @@ fn handle_request_inner(method: String, path: String, headers: Map, body: String
         ) + badge_css + success_body + page_close()
     }
 
+    // ── Downloads ─────────────────────────────────────────────────────────────
+    // Public installer page. macOS ships today; Windows/Linux linked but marked
+    // coming soon. Live-rendered like /about (page shell wraps downloads_page()).
+    if str_starts_with(path, "/downloads") {
+        let dl_title: String = "Download Neuron - macOS, Windows & Linux"
+        let dl_desc: String = "Download Neuron for macOS. Your AI that remembers you, running on your own machine. Windows and Linux support is on the way."
+        return page_open_seo(dl_title, dl_desc, "/downloads", dl_desc, "false") + downloads_page() + page_close()
+    }
+
     // ── Account dashboard ─────────────────────────────────────────────────────
     // Use prefix match so OAuth/email-confirmation redirects with query strings still hit.
     if str_starts_with(path, "/account") {