feat: ship a web app manifest and home-screen icons

Adding the app to a phone home screen produced a blank tile: index.html
linked only an SVG favicon, and Safari ignores SVG for apple-touch-icon,
substituting a screenshot of the page. There was no manifest at all, so
Android had nothing to install against either.

Generate the rasters from public/logo.svg with sharp, which the repo
already carries. The output is committed so a plain `vite build` needs no
image toolchain; re-run scripts/generate-icons.mjs after changing the
logo. The logo's viewBox is wider than it is tall, so each icon centres
the mark on a square #0a0a0f canvas rather than stretching it, inset to
78% normally and 54% for the maskable variant to survive Android's
circular crop.

The SVG favicon stays as the browser tab icon.

public/manifest.webmanifest

@@ -0,0 +1,21 @@
+{
+  "name": "God's Eye View",
+  "short_name": "God's Eye",
+  "description": "Live spatial intelligence in the browser.",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#0a0a0f",
+  "theme_color": "#0a0a0f",
+  "icons": [
+    { "src": "/icons/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/icons/icon-512.png", "sizes": "512x512", "type": "image/png" },
+    {
+      "src": "/icons/icon-512-maskable.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ]
+}