swarm UI: delete linked accounts — R2 fixes

Addresses argus review comment 90297 on PR #4899:

- swarm-controller/README.md: list the three DELETE routes (including
  matrix's ?revoke=true) beside the PUT/GET ones already documented.
- LinkedAccounts.tsx: a delete answering 404 means the account is
  already gone, so treat it as the delete's end state — re-fetch and
  close the dialog instead of showing an error.
- matrix_account.rs: matrix_logout treats a 401 M_UNKNOWN_TOKEN as the
  token already being revoked and proceeds with the delete; every
  other logout failure still keeps the account. Adds unit tests and
  updates docs/swarm/ui.md to match.

frontend/packages/swarm-ui/src/pages/agents/LinkedAccounts.tsx

@@ -96,7 +96,9 @@ export function LinkedAccounts({
         deleteTarget.kind === "matrix" && revoke ? `${url}?revoke=true` : url,
         { method: "DELETE" },
       );
-      if (!r.ok) {
+      // A 404 means the account the dialog is about is already gone — the
+      // end state the delete wanted — so it closes the same as a success.
+      if (!r.ok && r.status !== 404) {
         setDeleteError(await readApiError(r));
         return;
       }