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

templates/index.html

@@ -4,6 +4,11 @@
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <title>SIMPLE MINING — {{ bp }}</title>
+  <link rel="icon" type="image/svg+xml" href="{{ url_for('static', filename='favicon.svg') }}">
+  <link rel="icon" type="image/png" sizes="32x32" href="{{ url_for('static', filename='favicon-32.png') }}">
+  <link rel="icon" type="image/png" sizes="16x16" href="{{ url_for('static', filename='favicon-16.png') }}">
+  <link rel="icon" type="image/x-icon" href="{{ url_for('static', filename='favicon.ico') }}">
+  <link rel="apple-touch-icon" sizes="192x192" href="{{ url_for('static', filename='icon-192.png') }}">
   <link rel="preconnect" href="https://fonts.googleapis.com">
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
   <link href="https://fonts.googleapis.com/css2?family=Saira+Condensed:wght@400&family=EB+Garamond:ital,wght@0,400;1,400&family=JetBrains+Mono:wght@400&display=swap" rel="stylesheet">