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/tests/accounts/test_discovery.py

@@ -37,10 +37,14 @@ AUTOCONFIG_XML = """<?xml version="1.0"?>
 def _skip_ssrf(monkeypatch) -> None:  # type: ignore[no-untyped-def]
     """Bypass the real DNS/public-IP check so respx can intercept the fetch."""
 
-    async def ok(_host: str) -> None:
-        return None
+    async def ok(_host: str) -> list[str]:
+        return ["93.184.216.34"]
+
+    async def unpinned(url: str) -> tuple[str, dict[str, str], dict[str, str]]:
+        return url, {}, {}  # respx matches on the host name, not the pinned IP
 
     monkeypatch.setattr(ssrf, "assert_host_is_public", ok)
+    monkeypatch.setattr(ssrf, "_pinned", unpinned)
 
 
 # --- XML parsing -------------------------------------------------------------