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.

swarm-controller/README.md

@@ -34,6 +34,9 @@ The swarm's control plane. The swarm UI and `swarmctl` are its clients.
   `.../forge-accounts/{label}`, `.../github-account`); distinct from the agent's own swarm-minted
   accounts above. `GET .../linked-accounts` lists them, plus the agent's own
   `main` matrix account, by kind, name and host, never with a credential.
+  Each of the three also takes a `DELETE`; matrix's also takes
+  `?revoke=true`, which logs the stored token out at its homeserver first and
+  leaves the account in place if that fails.
 - **Config PR status** — each agent's open config-repo PR, cached from forge
   webhooks (`GET /api/config-prs`, `/api/agents/{name}/config-pr`).
 - **Swarm-wide forge objects and webhooks** →