Set the Archive role from ihasmail, rather than describing it

#220 corrected the message and left it useless: it told you a folder needs
the Archive role on the server, which was true, and gave you nothing to do
about it here. Roles were shown in Folders settings and never settable.

Mailbox/set takes `role`. Confirmed live against 0.16.20 on 2026-09-02, as an
ordinary user through the proxy, with no admin API: setting role "archive" on
a folder that had none returned updated and the folder began working as the
Archive immediately. Stalwart parses the role names in SpecialUse::parse,
"archive" among them, refuses a second holder of a role, and refuses to move
the role of Inbox, Junk or Trash.

So the toast now carries the fix. "No Archive folder is set yet." with a
Create one that makes the folder and then completes the archiving that could
not happen -- rather than leaving someone to select the same messages again.

A folder already named Archive and carrying no role is adopted rather than
duplicated. That is the state #217 was reported from, and a second Archive
beside the first would be its own confusion. One named Archive that is really
the Sent folder is left alone: taking its role to fix archiving would break
sending.

Folders settings gains a Role column. Archive, Drafts and Sent are offered,
being the roles this client's behaviour depends on and the server will move;
Inbox, Junk and Trash show theirs and cannot change it, because 0.16.20
refuses. A role another folder holds is left out of the list rather than
offered and refused, so freeing it is a deliberate two steps.

The folder is created with the server's own name, never the localised one,
for the reason renaming already writes back the server's: a German session
must not create "Archiv" that an English one cannot find.

Closes #217 properly.

web/src/locales/ja.ts

@@ -967,7 +967,14 @@ export const catalog: Catalog = {
     "New all-day event on {date}": "{date} に終日の予定を作成",
     "New event at {time}": "{time} に予定を作成",
     "New identity": "新しい差出人",
-    "No Archive folder is set. A folder needs the Archive role on the server; naming it “Archive” is not enough.": "アーカイブフォルダーが設定されていません。サーバーでフォルダーに「Archive」ロールが必要です。名前をそうするだけでは不十分です。",
+    "No Archive folder is set yet.": "アーカイブフォルダーがまだ設定されていません。",
+    "Create one": "作成する",
+    "Could not set up an Archive folder: {error}": "アーカイブフォルダーを設定できませんでした: {error}",
+    "Role": "ロール",
+    "Drafts": "下書き",
+    "Sent": "送信済み",
+    "The server does not allow this role to be changed.": "サーバーはこのロールの変更を許可していません。",
+    "Folder role updated": "フォルダーのロールを更新しました",
     "No contacts yet": "連絡先がまだありません",
     "No longer shared": "共有を解除しました",
     "No matches": "一致するものがありません",