Delete all spam, and call folders what the server calls them

Junk Mail can now be emptied in one action, the way every other mail
client offers it: a banner across the top of the folder, and an item in
both the folder's right-click menu and the list's own menu.

The messages are destroyed rather than moved to Deleted Items. Routing
spam through the bin on its way out leaves you with the same problem in
a different folder, and "delete all spam" means gone everywhere else. So
the dialog says it before you commit, and there is no undo.

emptyMailbox already did the hard part -- walking a folder a page at a
time so it survives maxObjectsInSet, which a Deleted Items of 5192 once
did not. All that changed is which folders it will accept. The guard
stays in the store rather than living only in the menus, so a fourth
caller cannot empty the Inbox by asking nicely.

The three entry points share one helper, because three dialogs warning
about a permanent deletion in three slightly different ways is how one
of them ends up not warning at all. A folder with nothing in it offers
the item greyed out rather than hiding it, so it is where you expect it
to be next time.

Folder naming is fixed in the same commit because it changed the same
file, and because testing this is what surfaced it. Two problems, one
cause:

  - The mock called its folders "Trash" and "Sent". Stalwart's defaults
    follow the Exchange convention -- "Deleted Items", "Sent Items" --
    so anything built from a folder's name read differently against the
    mock than against a real server, and every screenshot in the README
    showed a folder list no user has.

  - Worse, and in shipping code: the undo toast took a hardcoded label
    in preference to the folder's actual name, so deleting a message
    announced "moved to Trash" on a server whose folder is called
    "Deleted Items", and reporting spam said "moved to Spam" where it is
    "Junk Mail". The one message whose job is saying where mail went was
    naming somewhere that does not exist. It now prefers the mailbox's
    own name and keeps the hardcoded word only as a fallback.

Verified against the mock: the banner appears only in Junk and only with
something to delete, the dialog counts and pluralises, the messages are
destroyed and Deleted Items stays empty afterwards, the banner
disappears once the folder is, the item greys out when empty, Archive is
offered neither, and Trash still says "Empty Deleted Items".

README.md

@@ -65,6 +65,7 @@ seconds of downtime with nothing lost.
 - Gmail-style three-pane layout (reading pane right/bottom/off, **drag-to-resize splitter** in both orientations, quick layout switch in the list menu), conversation view with collapsed messages and "show quoted text", dense/cozy/comfortable density, light/dark/system themes plus **ihasmail** — the palette from ihasmail.org, and what a new account starts on — each with accent colours over the top
 - Virtualised, infinitely-scrolling message list; multi-select (click, ⇧-click, ⌃-click), drag & drop to folders, right-click context menus, hover actions, Gmail keyboard shortcuts (`j/k`, `e`, `#`, `r/a/f`, `g i`, `/`, `?` …)
 - Archive / delete / spam / star / mark read / move / labels (IMAP keywords with colours) with **Undo**
+- **Delete all spam** — a banner across the top of Junk Mail, and an item in the folder's right-click menu and the list's own menu, that empties it in one action. The messages are destroyed rather than moved to Deleted Items, since routing spam through the bin leaves you with the same problem in another folder; the dialog says so before you commit, and there is no undo. Only Deleted Items and Junk Mail can be emptied this way — enforced in the store rather than merely hidden in the menus — and a folder that is already empty offers it greyed out
 - **"Filter messages like this…"** from the message context menu: creates a Sieve rule pre-filled from the sender/list (target folders can be created on the fly), and can **apply it immediately to the existing messages in the folder** (evaluated client-side, actions applied via JMAP)
 - Safe HTML rendering: DOMPurify sanitisation inside a Shadow DOM, **remote images blocked by default** with a per-sender allow-list and an optional **privacy image proxy** (like Gmail's)
 - Messages sit on a light card by default, untouched as the sender designed them. *Appearance › Apply the theme to messages too* lets them follow the app's light/dark theme instead — plain-text mail always does, and with the option on so does HTML mail that brings no colours of its own; mail that styles itself is still left alone