feat: add logo, PWA manifest, favicon

- Geometric Companion logo: rounded head, blue outline, green eyes,
  green antenna tip. SVG for crisp rendering at any size.
- Logo appears in header next to site name
- PWA manifest (manifest.json) with 192px and 512px PNG icons
- Favicon (favicon.ico) for browser tab
- Apple touch icon for iOS home screen
- README: logo replacement instructions for whitelabeling

To replace the logo for whitelabeling, just swap frontend/logo.svg
and the icon PNGs. Update <link> tags in index.html if filename changes.

manifest.json

@@ -0,0 +1,26 @@
+{
+  "name": "Protocol Droid",
+  "short_name": "Protocol Droid",
+  "description": "A tool for authoring, sharing, and curating social protocols",
+  "start_url": "./",
+  "display": "standalone",
+  "background_color": "#0a1929",
+  "theme_color": "#0a1929",
+  "icons": [
+    {
+      "src": "frontend/logo.svg",
+      "sizes": "any",
+      "type": "image/svg+xml"
+    },
+    {
+      "src": "frontend/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "frontend/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    }
+  ]
+}