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>

docs/nas-install.md

@@ -569,6 +569,24 @@ Session cookies get the `Secure` and `__Host-` treatment automatically as soon
 as the app sees an HTTPS request: nginx forwards `X-Forwarded-Proto`, and the
 API is started with `--proxy-headers`. Nothing to configure.
 
+**Plain HTTP and HSTS.** If DSM also has a rule for **HTTP** / port 80 to the
+same hostname, keep it: the web container answers any request DSM forwarded
+over plain HTTP with a `301` to `https://` (it recognises one by DSM's
+`X-Forwarded-Proto: http`), and sends `Strict-Transport-Security` on HTTPS
+responses. Leave DSM's own *Enable HSTS* box unticked — the container already
+sends the header. Opening `http://NAS:8088` on the LAN is not redirected
+(no reverse proxy, no such header).
+
+**Security headers** (CSP, framing, `nosniff`, referrer, HSTS) are set by the
+web container on every response — `docker/nginx-security.conf`. Don't add
+Custom Headers for them in DSM: two differing CSPs are both enforced.
+
+**Breached-password check.** New passwords (sign-up, reset) are checked
+against Have I Been Pwned's range API — only the first 5 hex digits of the
+password's SHA-1 leave the server. The API container needs outbound HTTPS to
+`api.pwnedpasswords.com`; when it cannot reach it the check is skipped, never
+fatal. `PASSWORD_BREACH_CHECK=false` turns it off.
+
 ### 5.3 Installing as an app, and notifications about new mail
 
 Over HTTPS the web app is installable (Chrome/Edge: the install icon in the