Folder names follow the language, and three bugs that found

Answering "can we ask Stalwart to serve German folder names": no, and it would
not help if we could. The account locale exists in `x:AccountSettings`, and
ihasmail already reads it -- that is what "Your mail server reports German"
comes from -- but writing it needs `sysAccountSettingsSet`, which the built-in
user role does not carry; only an admin could. And even then it would change
nothing, because folder names are stored data written once when the account is
provisioned. No server renames them afterwards; every other client has them
mapped.

The role is the way through. JMAP tags the standard folders and ihasmail
already trusts the role over the name everywhere it matters, so the *displayed*
name can follow the interface language with nothing written to the server. A
folder somebody made and called "Newsletters" keeps that name: those are their
words, and translating them would name a folder they never created.

The cost is real and worth stating: Thunderbird on the same account still shows
"Deleted Items", because that is what the folder is called. Inside ihasmail it
stays consistent -- everything that names a folder goes through one function,
including the "moved to …" toast, which exists precisely so that message does
not name somewhere the reader cannot find. Renaming still edits the server's
own name, never the localised one.

Three things fell out of it.

The message list refreshed for ever after a language change, which is the one
somebody noticed. The root keys its tree on the language version, so a publish
remounts everything; remounting re-runs the effect that loads the account's
settings, which calls applyLang, which called setCatalog again -- with an
identical tag and an identical catalogue -- and publishing that non-change went
round again. setCatalog now returns early when nothing changed. Measured
rather than assumed: three consecutive five-second windows with no JMAP calls
at all, against a pre-change count that never settled.

Calendar months and weekdays stayed English, because formatting locale and
interface language are separate settings and only the first feeds Intl.
Keeping them separate is right -- German dates with an English interface is a
real preference -- but somebody who picks German and is shown "September" has
not got what they asked for. A chosen interface language now joins the
*automatic* chain ahead of the server and the browser. Setting a formatting
locale explicitly still wins, and English is not counted, so an English
interface on a German browser keeps German dates exactly as before.

And the Archive folder read "Archivieren", which is the verb. English uses one
word for the button and the folder; German does not, and neither does
"Important", which is also a priority tag. tc(context, source) keys the
catalogue on both and falls back to the plain English, which was right in
English all along -- the gettext approach, including the control character as
separator so no real string can collide.

The catalogue checker needed teaching about tc() twice: first it reported the
eight contextual entries as stale, then it asked for the plain fallbacks as
though they were a second obligation. A check that reports work which does not
exist gets switched off, which is worse than not having one.

web/src/lib/i18n.ts

@@ -61,6 +61,30 @@ export function t(source: string, vars?: Vars): string {
   return interpolate(current.strings[source] ?? source, vars);
 }
 
+/**
+ * Translate where the English word is doing two jobs.
+ *
+ * English-as-key has one real weakness and this is it: "Archive" is the button
+ * that archives a message and the folder the message lands in, and German
+ * needs "Archivieren" for the first and "Archiv" for the second. One key
+ * cannot hold both. "Important" is the same — a priority tag and a folder.
+ *
+ * So a context can be given, and the lookup becomes context + source while the
+ * fallback stays the plain English. A translator sees the context and knows
+ * which sense to render; a catalogue that has not got round to it still
+ * renders the English word, which was right in English all along.
+ *
+ * The separator is a control character rather than a punctuation mark, which
+ * is the gettext convention and for the same reason: no English string can
+ * contain it by accident.
+ */
+export const CONTEXT_SEPARATOR = "\u0004";
+
+export function tc(context: string, source: string, vars?: Vars): string {
+  const keyed = current.strings[`${context}${CONTEXT_SEPARATOR}${source}`];
+  return interpolate(keyed ?? current.strings[source] ?? source, vars);
+}
+
 /**
  * Translate a counted thing.
  *
@@ -119,6 +143,12 @@ export function tNode(source: string, parts: Record<string, ReactNode>, vars?: V
   return out;
 }
 
+/** Subscribe to catalogue changes without React. Used by the tests. */
+export function subscribeForTest(fn: () => void): () => void {
+  listeners.add(fn);
+  return () => void listeners.delete(fn);
+}
+
 /** The language in force, for anything that needs the tag itself. */
 export function currentLanguage(): string {
   return currentTag;
@@ -132,6 +162,23 @@ export function currentLanguage(): string {
  * language's rules against another's forms.
  */
 export function setCatalog(tag: string, catalog: Catalog): void {
+  /*
+   * Publishing only when something actually changed is not an optimisation
+   * here, it is the thing that stops an infinite loop.
+   *
+   * The root keys its tree on the language version, so a publish remounts
+   * everything. Remounting re-runs the effect that fetches the account's
+   * settings file, which calls `hydrate`, which calls `applyLang`, which lands
+   * back here -- with the identical tag and the identical catalogue. Publishing
+   * that non-change bumped the version again and went round for ever: the
+   * message list refetched on every pass, which is what it looked like from
+   * the outside.
+   *
+   * Reference equality is enough. `EMPTY` is a module constant and a
+   * dynamically imported catalogue is cached, so the same language really does
+   * hand back the same object.
+   */
+  if (currentTag === tag && current === catalog) return;
   currentTag = tag;
   current = catalog;
   publish();