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/store/__tests__/archive-by-date.test.ts

@@ -178,13 +178,13 @@ describe("archiveByDate", () => {
     expect(s.moves).toEqual([]);
     expect(messages()[0]).toContain("No Archive folder");
     /*
-     * What it says, not just that it complains. The folder is found by its
-     * special-use role and by nothing else, so telling someone to create a
-     * folder *named* "Archive" sent them round a loop that could not end --
-     * naming a folder does not give it the role, and ihasmail cannot assign
-     * one. Issue #217.
+     * And it offers to fix it. The folder is found by its special-use role and
+     * by nothing else, so telling someone to create a folder *named* "Archive"
+     * sent them round a loop that could not end. ihasmail can set the role
+     * itself, so the toast carries the action rather than the explanation.
+     * Issue #217.
      */
-    expect(messages()[0]).toContain("role");
+    expect(useToasts.getState().toasts[0]?.action?.label).toBeTruthy();
     expect(messages()[0]).not.toMatch(/Create one named/);
   });