Go to a folder by name, with g then o

Requested in #233. The `g` shortcuts cover the handful of folders every
account has -- inbox, sent, drafts -- and nothing reaches the dozens a Sieve
rule fills, which is where somebody with a real folder tree spends their time.
`g o` opens the picker, you type part of a name, and you are there.

The picker is the one the move action already uses, with one difference that
only shows up on shared mail: it selected folders by `mayAddItems`, which is
right for a destination and wrong for a place to go. A shared folder you may
read but not file into is somewhere you can visit. The right is now a
parameter, named for what it is asking rather than for which caller wants it.

Hosted in AppShell rather than in the mail view, because the `g` shortcuts are
global and the mail view is not mounted to hear about it -- pressing this from
the calendar should still take you to a folder, and now does.

`o` on its own opens a conversation and does not clash: a pending prefix is
tried before a bare key. That was already true and nothing said so, so there
are now five tests for the sequence machinery -- including that an abandoned
prefix costs the prefix and not the keystroke after it, which is the nicer
behaviour of the two and was undocumented.

Checked in a browser against the mock: opened from the calendar, filtered to a
nested folder, landed on it, and `o` still opened a conversation afterwards.

Closes #233.

web/src/locales/pt-BR.ts

@@ -50,6 +50,7 @@ import type { Catalog } from "@/lib/i18n";
  */
 export const catalog: Catalog = {
   strings: {
+    "Go to folder…": "Ir para a pasta…",
     "Set for everyone here. You cannot change this.": "Definido para todos aqui. Você não pode alterar isto.",
     "Export iCAL file": "Exportar arquivo iCAL",
     "Could not export this calendar: {error}": "Não foi possível exportar esta agenda: {error}",