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/providers/test_parsing.py

@@ -100,7 +100,15 @@ def test_parse_search_uids() -> None:
 
 def test_parse_vanished() -> None:
     # aioimaplib strips the leading "* " before buffering the line.
-    assert parse_vanished([b"VANISHED (EARLIER) 300:305,405"]) == [300, 301, 302, 303, 304, 305, 405]  # noqa: E501
+    assert parse_vanished([b"VANISHED (EARLIER) 300:305,405"]) == [
+        300,
+        301,
+        302,
+        303,
+        304,
+        305,
+        405,
+    ]  # noqa: E501
     # Live-expunge form (no EARLIER) and a descending range are both handled.
     assert parse_vanished([b"VANISHED 41", b"VANISHED (EARLIER) 5:3"]) == [41, 3, 4, 5]
     # Non-VANISHED lines are ignored; non-bytes entries are skipped.