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.

Cargo.toml

@@ -4,7 +4,7 @@ version = "0.1.0"
 edition = "2021"
 # The Proton crates set the floor.
 rust-version = "1.96"
-description = "A terminal-themed personal diary for the browser"
+description = "A private, terminal-themed workspace for the browser: diary, notes, boards and files"
 
 [dependencies]
 axum = { version = "0.8", features = ["multipart", "macros"] }