brand: an icon, ~/ and a block cursor

The app was a ring and a dot, drawn in CSS in the bar and inlined as a data
URL for the favicon. It now has an icon of its own: the `~/` of the prompt it
is named after, with the cursor waiting beside it in the alarm colour of the
theme. It is the favicon, a full-bleed touch icon for a phone's home screen,
and the mark in the bar, on the login and on the share page, served from
`web/public` so all of them are the same file.

The banner and a 512 px copy in docs/images are for the README and the wiki.
The crate description says what the workspace is now, not the diary it was.

web/index.html

@@ -6,7 +6,8 @@
     <meta name="color-scheme" content="dark" />
     <meta name="theme-color" content="#0c0d0b" />
     <title>~/workspace</title>
-    <link rel="icon" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 64 64'%3E%3Crect width='64' height='64' rx='14' fill='%230c0d0b'/%3E%3Ccircle cx='32' cy='32' r='15' fill='none' stroke='%23eef1ea' stroke-width='3'/%3E%3Ccircle cx='32' cy='32' r='7' fill='%23eef1ea'/%3E%3C/svg%3E" />
+    <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
+    <link rel="apple-touch-icon" href="/apple-touch-icon.png" />
   </head>
   <body>
     <div id="app"></div>