Landing: add marketing page at /, move the app to /app/

Multi-page Vite build: index.html serves a new standalone landing
(src/landing), app/index.html serves the app at /app/. The PWA service
worker and manifest are scoped to /app/, so the landing stays
uncontrolled and always-fresh; hash view-state routing is unchanged.

The landing reuses src/core and the chart-renderer service only (never
stores/orchestration/modals/components) and lazy-loads Vega. Its demos
drive the real core: a hero snippet switcher over CHART_EXAMPLES, an
interactive Chart Builder over a new profiled core fixture
(sample-dataset), and a Theme Builder gallery with three from-scratch
custom themes plus built-in presets.

vite.config.ts

@@ -20,6 +20,13 @@ export default defineConfig({
   },
   build: {
     rollupOptions: {
+      // Multi-page: the marketing landing is served at `/` (index.html → the
+      // lightweight `src/landing` entry); the app at `/app/` (app/index.html →
+      // src/main.tsx). The app is hash-routed, so it runs unchanged under /app/.
+      input: {
+        main: fileURLToPath(new URL('./index.html', import.meta.url)),
+        app: fileURLToPath(new URL('./app/index.html', import.meta.url)),
+      },
       output: {
         // Split the heavy vendors so the chunk graph stays legible and the PWA
         // can precache/update them independently of app code. Monaco and the
@@ -35,10 +42,18 @@ export default defineConfig({
     react(),
     VitePWA({
       registerType: 'prompt',
+      // The app lives at /app/; scope the service worker there so it never
+      // controls the marketing landing at / (which stays uncontrolled and
+      // always-fresh). A /sw.js narrowing its scope to /app/ needs no
+      // Service-Worker-Allowed header — narrowing is always permitted.
+      scope: '/app/',
       includeAssets: ['favicon.svg', 'icon-maskable.svg', 'icon-mono.svg', 'apple-touch-icon.png'],
       manifest: {
+        id: '/app/',
         name: 'Astrolabe',
         short_name: 'Astrolabe',
+        scope: '/app/',
+        start_url: '/app/',
         description: 'A browser-based snippet manager for Vega-Lite visualizations.',
         // theme_color tints the OS/browser chrome; background_color is the splash
         // behind the icon. theme_color is the app accent (`--accent`, light theme);
@@ -62,6 +77,9 @@ export default defineConfig({
         ],
       },
       workbox: {
+        // Client-side navigations under /app/ fall back to the app shell when
+        // offline; the SW's /app/ scope means / (the landing) is never matched.
+        navigateFallback: '/app/index.html',
         // Precache the app, plus the font woff2 the offline-first UI and the
         // common chart path need:
         //  - the bare `latin` subset of every family (`*-latin-[0-9]*` — the