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/datetime.ts

@@ -24,6 +24,28 @@ export interface DateTimePrefs {
 
 const DEFAULT_PREFS: DateTimePrefs = { locale: "", dateFormat: "auto", timeFormat: "auto" };
 
+/**
+ * The interface language, when one has been chosen over the default.
+ *
+ * Formatting and interface language are separate settings on purpose -- German
+ * dates with an English interface is a real preference. But somebody who picks
+ * German and is then shown "September" and "Monday" has not got what they
+ * asked for: choosing a language *is* a statement about language, and month
+ * names are language.
+ *
+ * So it joins the automatic chain, ahead of the server and the browser, and
+ * only while the formatting locale is left on "Automatic". Setting one
+ * explicitly still wins over everything, which is what that setting is for.
+ * English is not counted, because it is the default nobody has to choose --
+ * an English interface on a German browser should keep German dates, as it
+ * always has.
+ */
+let uiLanguage: string | null = null;
+
+export function setUiLanguageForFormatting(tag: string | null | undefined): void {
+  uiLanguage = tag && tag !== "en" ? tag : null;
+}
+
 let prefs: DateTimePrefs = DEFAULT_PREFS;
 let serverLocale: string | null = null;
 
@@ -90,9 +112,9 @@ export function normalizeLocale(raw: string | null | undefined): string | null {
   }
 }
 
-/** The locale Intl should use: explicit choice → server → browser default. */
+/** Explicit choice → chosen interface language → server → browser default. */
 export function resolvedLocale(): string | undefined {
-  return prefs.locale || serverLocale || undefined;
+  return prefs.locale || uiLanguage || serverLocale || undefined;
 }
 
 /** Where the effective locale came from — used to label the "Automatic" option. */
@@ -524,7 +546,7 @@ let optionsExtras = "";
  * outside the generated list is still selectable).
  */
 export function localeOptions(): LocaleOption[] {
-  const extras = `${serverLocale ?? ""}|${prefs.locale}`;
+  const extras = `${serverLocale ?? ""}|${prefs.locale}|${uiLanguage ?? ""}`;
   if (optionsCache && optionsExtras === extras) return optionsCache;
   const tags = new Set<string>(LOCALE_TAGS);
   if (serverLocale) tags.add(serverLocale);