feat: crypto mining isometric cube favicon + icons

- static/favicon.svg: isometric 3D cube, white on black, DESIGN.md surface tones
- static/favicon-16.png, favicon-32.png, icon-192.png, icon-512.png, favicon.ico
- index.html + history.html: <head> links for SVG, PNG, ICO, apple-touch-icon

static/favicon.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 200" width="200" height="200">
+  <rect width="200" height="200" fill="#000000"/>
+  <polygon points="100,30 170,70 100,110 30,70" fill="#1f1f1f" stroke="#ffffff" stroke-width="2"/>
+  <polygon points="30,70 100,110 100,170 30,130" fill="#141414" stroke="#ffffff" stroke-width="2"/>
+  <polygon points="100,110 170,70 170,130 100,170" fill="#0d0d0d" stroke="#ffffff" stroke-width="2"/>
+</svg>