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.

frontend/logo.svg

@@ -0,0 +1,13 @@
+<svg width="128" height="128" viewBox="0 0 120 140" xmlns="http://www.w3.org/2000/svg">
+  <line x1="60" y1="5" x2="60" y2="20" stroke="#6db5ff" stroke-width="2.5"/>
+  <circle cx="60" cy="5" r="3.5" fill="#00ff88"/>
+  <rect x="30" y="20" width="60" height="65" rx="12" fill="none" stroke="#6db5ff" stroke-width="2.5"/>
+  <circle cx="45" cy="42" r="6.5" fill="#0a1929" stroke="#6db5ff" stroke-width="2"/>
+  <circle cx="45" cy="42" r="3" fill="#00ff88"/>
+  <circle cx="75" cy="42" r="6.5" fill="#0a1929" stroke="#6db5ff" stroke-width="2"/>
+  <circle cx="75" cy="42" r="3" fill="#00ff88"/>
+  <rect x="48" y="60" width="24" height="3" rx="1" fill="#6db5ff" opacity="0.7"/>
+  <rect x="50" y="66" width="20" height="3" rx="1" fill="#6db5ff" opacity="0.4"/>
+  <rect x="48" y="85" width="24" height="8" fill="none" stroke="#6db5ff" stroke-width="2"/>
+  <path d="M 25 93 Q 60 100 95 93" fill="none" stroke="#6db5ff" stroke-width="2.5"/>
+</svg>