Security hardening from the assessment; testing fixes

Security (ultimo-security-report.md):
- Security headers on every response, the app shell included: they lived at
  server level and nginx dropped them in any location with its own header.
  CSP, framing, nosniff, referrer, permissions (docker/nginx-security.conf).
- Plain HTTP forwarded by the reverse proxy is redirected to HTTPS; HSTS on
  HTTPS responses. server_tokens off.
- Cross-site state-changing /api requests refused (login CSRF), WebSocket too.
- Validation errors in the usual error shape, without the request model.
- New-password policy: max 128, common/own-address/repeated refused,
  breached-password check (HIBP k-anonymity, fails open).
- Message HTML: links keep only http(s)/mailto (sanitizer v6); the HTML
  endpoint carries its own no-script CSP.
- Outbound fetches pinned to the checked IP (DNS rebinding); external
  IMAP/SMTP servers re-checked before each connection; CGNAT and mapped
  addresses counted as non-public.

Fixes from testing:
- IDLE loop catches up before idling: mail that came while the worker was
  away no longer waits up to 10 minutes.
- Restore from Spam/Trash/Archive moves only the removed messages, not your
  reply in Sent.
- Narrow desktop list stacks sender/subject/snippet (subject was squeezed
  to nothing at the default width), with hover actions.
- Phone: wide emails scale to fit the screen.
- Singular forms ("1 message"); no zip button for a single attachment;
  app passwords no longer claim "never used" for mail apps.
- ruff format across the backend.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/ws.py

@@ -18,6 +18,7 @@ from app.events import get_event_bus
 from app.identity.cookies import SESSION_COOKIE_INSECURE, SESSION_COOKIE_SECURE
 from app.identity.sessions import resolve_session
 from app.redis_client import get_redis
+from app.security.cross_site import is_cross_site
 from app.sync import leases
 
 logger = logging.getLogger(__name__)
@@ -27,6 +28,12 @@ router = APIRouter()
 
 @router.websocket("/ws")
 async def ws_endpoint(websocket: WebSocket) -> None:
+    # Only our own pages may listen in: a socket another site opens is refused
+    # even if the browser sent the session cookie with it (cross-site WebSocket
+    # hijacking; SameSite=Lax already stops current browsers from sending it).
+    if is_cross_site(websocket.headers):
+        await websocket.close(code=1008)
+        return
     token = websocket.cookies.get(SESSION_COOKIE_SECURE) or websocket.cookies.get(
         SESSION_COOKIE_INSECURE, ""
     )