Cannot leave a community whose relay already ended my membership (403 → 'Relay session is terminal')

**Buzz Desktop version:** 0.5.19 (macOS, Darwin 25.6.0)

## Summary

When a relay has already ended my membership server-side, the desktop app cannot remove that community locally. The "Leave community" flow requires the relay to accept the leave event first (per #3621), but the relay refuses the connection entirely — so the leave can never succeed and the dead community is stuck in the community switcher permanently.

## Steps to reproduce

1. Be a member of a community, then have your membership ended on the relay side (removed by the owner / membership expired).
2. The sidebar for that community shows: `relay returned 403 Forbidden: You must be a relay member to access this relay`.
3. Open the community switcher → hover the community → **Leave community**.
4. The action is blocked with: `Relay session is terminal; cannot reconnect.`

## Expected

If the relay reports the session is terminal / membership already ended (403 `relay_membership_required`), local removal should proceed anyway — there is no membership left to end, so requiring relay acceptance is a chicken-and-egg dead end.

## Actual

"Leave community" never completes; the community cannot be removed from the local list. Updating to 0.5.20 does not change this (no related fix in the release notes).

## Notes

#3621 deliberately keeps the community in place on relay rejection, which is correct for transient errors — but a terminal/403-membership response is not transient and should be treated as "already left".

Start with the desktop app's Leave community flow and the handling of relay 403 responses, especially the terminal-session path described in the issue. Verify the behavior against the #3621 rejection handling; done means a terminal or 403 membership response removes the community locally while transient relay failures remain blocked.

Written by the indexing model from the issue text.