Archive into a dated folder

Archiving put everything in one folder, so an Archive that has been
collecting for years is a single flat list with no way to narrow it except
search.

Archive by year and Archive by month file into Archive/2026 and
Archive/2026/09, creating the folders as needed and reusing them after
that, including folders made by hand or by another client.

The names are numeric and zero-padded rather than month names, because
these are real server-side mailboxes rather than anything of ihasmail's.
Every other client sees them: a folder created as "September" by someone
reading in English stays "September" for the same account read in
Japanese, since the name is stored and not translated. And 09 sorts
between 08 and 10 where a name does not.

The date is read in the reader's own timezone rather than UTC so it agrees
with the date shown against the message in the list. A message that
arrived at 00:30 UTC on 1 September is dated 31 August in New York, and
filing it under 09 while the list says August would be the app
disagreeing with itself. A message whose date cannot be read goes to
Archive itself rather than to a folder named after a guess.

A selection spanning two months is two destinations, not one. The moves
are made silently and one toast names where everything went -- the folder
where there is a single answer, the count where there is not -- because
each group raising its own toast with its own Undo would mean undoing a
third of a move. One Undo restores the whole selection to wherever each
message came from, captured before anything moved.

The menu labels name the destination where there is one, so it reads
"Archive to 2026/09" rather than describing the rule, and falls back to
"Archive by month" for a selection with no single answer.

FEATURES.md

@@ -150,6 +150,21 @@ Archive, delete, spam, star, mark read/unread, move and label all offer **Undo**
 in the toast that follows, and the undo restores the previous state rather than
 guessing at an inverse.
 
+**Archive by date** files into `Archive/<year>` or `Archive/<year>/<month>`,
+creating the folders as needed and reusing them after that — including ones
+made by hand or by another client. The names are numeric and zero-padded
+(`2026`, `2026/09`) rather than month names, because these are real server-side
+mailboxes: every other client sees them, a folder created as "September" by
+someone reading in English stays "September" for the same account read in
+Japanese, and `09` sorts between `08` and `10` where a name does not. The date
+is read in the reader's own timezone, so it agrees with the date shown against
+the message in the list.
+
+A selection spanning two months is two destinations, not one, and both are
+written; the menu names the folder where there is a single answer and describes
+the rule where there is not, and the toast afterwards says how many folders it
+touched. One Undo puts the whole selection back wherever it came from.
+
 `Delete` moves to the bin. **Empty** destroys, and is offered only on Deleted
 Items and Junk Mail — enforced where the action happens, not merely hidden in
 the menu. Emptying Junk destroys rather than moving to the bin, because routing