feat: add layout components (Navbar, Footer, PageContainer, AppShell)

- Navbar: responsive nav with ShieldAI logo, nav links, auth buttons,
  mobile hamburger menu, theme toggle, scroll-aware glass effect
- Footer: multi-column responsive layout with product/company/resources/
  legal links, social icons, copyright bar
- PageContainer: centered wrapper with max-w-7xl and responsive padding
- AppShell: root layout composing Navbar + main + Footer with dot-grid
  background and MetaProvider
- useAuth stub hook for future auth integration (task 23)
- Wire AppShell into app.tsx as Router root
- Unit tests for PageContainer and useAuth

web/src/app.tsx

@@ -3,19 +3,17 @@ import { Router } from "@solidjs/router";
 import { FileRoutes } from "@solidjs/start/router";
 import { Suspense } from "solid-js";
 import { useTheme } from "./lib/theme";
+import { AppShell } from "./components/layout";
 import "./app.css";
 
 export default function App() {
   useTheme();
   return (
     <Router
-      root={props => (
-        <MetaProvider>
-          <Title>SolidStart - Basic</Title>
-          <a href="/">Index</a>
-          <a href="/about">About</a>
+      root={(props) => (
+        <AppShell>
           <Suspense>{props.children}</Suspense>
-        </MetaProvider>
+        </AppShell>
       )}
     >
       <FileRoutes />