keys: files and uploads answer the keyboard

`gx` opens the file under the cursor in a tab of its own, as the browser
shows it. `:download` (`:dl`) downloads it, after asking for the vault when
the file is sealed; on the open document it is `:export`. `:source` turns a
markdown file between rendered and as written, which before had only a
button. A sealed file asks for the vault before `gx` instead of after, so
the browser does not block the late tab as a popup.

The media list gets a cursor: `j`/`k` walk it, `i` or `Enter` embeds the
highlighted upload at the cursor while writing, `y` copies its address and
`x` deletes it. Deleting asks through the `[y/N]` prompt, which names what
would break, in place of the browser's confirm(): that one was the last
modal on the page and needed the pointer to dismiss. A footer lists the
keys, as the agenda's does.

web/src/lib/Help.svelte

@@ -16,6 +16,7 @@
     ['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'],
+    ['gx', 'open the file under the cursor in a tab of its own'],
     ['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'],