editor: one editor, and everything written saves itself

TextEditor.svelte is folded into Editor.svelte. A card's description and a
note in the scratchpad are written in the same editor as a document, with
the same `[[` completion, and kept the same way: a new Writing (writing.svelte.ts)
stashes each change in localStorage and sends it to the server once the
typing stops, retrying a server that cannot be reached and recovering a
draft a dead tab left behind.

The keys out are the same everywhere: `:w` saves sooner, `:q` or Esc in
normal mode leaves with what is waiting written first, and stays where that
fails; `:q!` lets go of what is not saved. The store's describing and
scratch-editing flags are getters over the one `writing`.

docs/wiki/Boards-and-Agenda.md

@@ -93,9 +93,10 @@ as they do on the board.
 | `q`, `Esc` | close the back, back to the board |
 
 The description is markdown, written in the same editor as a document, vim
-and all: `:w` writes it to the card, `:q` or `Esc` in normal mode leaves, and
-`:q!` throws away what was not written. Leaving the tile while writing keeps
-what is written. A fenced block in a language is
+and all. It saves itself as the typing stops, as a document does; `:q` or `Esc`
+in normal mode leaves, and `:q!` lets go of what is not saved yet. Leaving the
+tile while writing keeps what is written. `[[` links a document by its title,
+and the card then shows among that document's cards. A fenced block in a language is
 coloured the way nvim colours it, written and read.
 
 Labels are lower case, and each name keeps its colour on every card and every