keys: links are followed without the pointer

`F` puts a letter or two over every link and button in view, vimium-style,
and typing one does what a click on it would. `f` was already fullscreen, so
the hints live on the other key. The labels start from the home row and are
all one length, so a label is done the moment it is typed. Esc, a key that
matches no label, or anything that scrolls the page puts them away. In the
file pane they reach the unlock and keystore buttons too.

A workspace link (`/n/…`) opens in place, as a click on it already did. Any
other link gets a tab of its own instead of taking the workspace with it.

In the editor, `gf` follows the link under the cursor, or the next one on
the line. A workspace link saves the text and opens that document. `gx`
opens the link in a new tab, as vim hands it to the browser. Both read
markdown links, embeds, `<…>` autolinks and bare addresses.

web/src/lib/Help.svelte

@@ -14,6 +14,8 @@
     ['Ctrl-D / Ctrl-U', 'half a page down / up the open document'],
     [']] / [[', 'next / previous heading of the document — in the editor too'],
     ['gO', 'the headings as a list: j/k, Enter jumps there'],
+    ['F', 'hints over the links and buttons in view — type one to follow it'],
+    ['gf / gx', 'in the editor: follow the link under the cursor / open it in a tab'],
     ['Enter · l', 'open a document, or enter a folder — on an open keystore or sealed file, Enter asks for its password'],
     ['h', 'from the document to the list, from the list up a level'],
     ['o', 'new document here, straight into insert mode'],