Select a whole folder, not just the rows that are loaded

The header checkbox selected the loaded page. On a folder of ten thousand
that is fifty of them, and the only way to act on the rest was to scroll
until they loaded and tick again.

A line now offers the rest by name once the page is selected, and taking
it is a separate press. A checkbox that silently meant ten thousand when
the screen shows fifty would be the worst of both, so each option says
what it actually covers.

The wider selection is a query rather than a list of ids. What it reaches
is resolved from the server when an action runs, walked a page at a time,
because a folder holds far more than one call returns and Email/set
refuses more ids than maxObjectsInSet in one go -- which setEmails already
chunks for. It resolves uncollapsed, unlike the list: "everything in this
folder" means every message rather than one per thread, and expanding
threads the way a click does is impossible here anyway, since that walks
loaded Email objects and these are the ones that were never loaded.

Two things this exposed.

Undo is now withheld once a move reaches messages that were never loaded.
It restores the folders each message was in, taken from what the browser
holds, and for an unloaded message that is nothing -- so the undo would
have written an empty mailboxIds and left the message in no folder at all,
which is worse than the move it was undoing. move() and archiveByDate()
both did this; both now drop the offer rather than restore something
wrong.

And an action consumes the wider selection. The optimistic paths cleared
the selected ids but not the flag, so the next action would have silently
reached the whole folder again.

web/src/styles/app.css

@@ -1401,3 +1401,6 @@ button.dp-open:disabled { cursor: default; opacity: .5; }
 
 /* The banner naming a sender from outside the organisation. */
 .remote-banner.external-banner { background: var(--warn-soft); border-color: var(--warn); }
+
+/* The line offering the whole folder once the page is selected. */
+.list-hint.select-all-hint { background: var(--accent-soft); color: var(--accent-soft-fg); }