Keep a settings change made before the first read, and wait for it

Two defects on the path that decides what language the app starts in.

**A change made before the settings file came back was thrown away.**
`queueSettingsPush` returned early while unarmed, dropping the value
instead of holding it, so a language picked in the second or so after a
page load was never written up: it survived until the next reload and no
further. That is a better account of "sometimes it takes several clicks"
than the remount race fixed in #160 — the click that stuck was one made
after the read had finished. Keeping it is safe because hydrate already
refuses to overwrite a key that is still queued.

Proof it was real: before this, no `ihasmail` folder was ever created in
the account's files, because the seed write never fired. After it, the
folder appears.

**Without a cached settings object the tree painted too early.** The
cache is not read on an untrusted device, and it is cleared by the
sign-out that every deploy causes, so in both cases the first frame is
the defaults — and the defaults are English. Anything computed in that
window is computed in the wrong language. The interface recovers, since
it is rebuilt when the catalogue lands, but a string emitted once does
not: this is why the stale-folder toast came out in English on an
otherwise German screen.

So without a cache the authenticated tree now waits for the account's
settings and their catalogue, which costs nothing — there was nothing
worth painting yet. With a cache it does not wait, and the first frame is
as quick as it was.

Neither fix makes the toast German yet: the account settings file is
neither written nor read successfully in the mock, and both failures are
swallowed. That is a third problem, and this commit does not touch it.

web/src/lib/storage.ts

@@ -107,6 +107,25 @@ export function loadJson<T>(key: string, fallback: T): T {
   }
 }
 
+/**
+ * Did `loadJson` have something to return, or did it hand back the fallback?
+ *
+ * The difference decides whether a first paint is worth anything. With a
+ * cached value the screen can be right immediately and the account's copy only
+ * has to correct it; without one -- an untrusted device, or the sign-out that
+ * every deploy causes -- the first paint is the defaults, and painting it
+ * before the account's settings arrive shows English to somebody who chose
+ * otherwise.
+ */
+export function hasCachedJson(key: string): boolean {
+  if (!trusted) return false;
+  try {
+    return localStorage.getItem(PREFIX + key) != null;
+  } catch {
+    return false;
+  }
+}
+
 export function loadRaw<T>(key: string, fallback: T): T {
   if (!trusted) return fallback;
   try {