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/security/cross_site.py

@@ -0,0 +1,91 @@
+"""Refuse state-changing API requests that another web site makes a browser send.
+
+The double-submit CSRF token (app/identity/deps.py ``require_csrf``) guards
+everything a signed-in user can change. The requests made before anyone is
+signed in — sign-in itself, sign-up, a password reset — have no token yet, so a
+hostile page could make a visitor's browser sign in to an account of the
+attacker's choosing (login CSRF): whatever the victim then writes, files or
+uploads lands in the attacker's account.
+
+Browsers say where a request comes from: ``Sec-Fetch-Site`` in every current
+browser, ``Origin`` on any cross-origin POST in older ones. A request another
+site started is refused here, before any route runs. Mail apps, calendar apps
+and servers send neither header and are not affected (DAV lives outside /api
+anyway).
+"""
+
+from __future__ import annotations
+
+import json
+from urllib.parse import urlsplit
+
+from starlette.datastructures import Headers
+from starlette.types import ASGIApp, Receive, Scope, Send
+
+from app.config import get_settings
+
+_SAFE_METHODS = frozenset({"GET", "HEAD", "OPTIONS"})
+_BODY = json.dumps(
+    {"error": {"code": "cross_site", "message": "request refused: sent from another site"}}
+).encode()
+
+
+def _hostname(value: str) -> str:
+    """``host[:port]`` -> ``host``, lower-cased (IPv6 brackets kept)."""
+    value = value.strip().lower()
+    if value.startswith("["):
+        return value.split("]", 1)[0] + "]"
+    return value.rsplit(":", 1)[0] if value.count(":") == 1 else value
+
+
+def is_cross_site(headers: Headers) -> bool:
+    """Whether a browser reports this request as started by another site."""
+    fetch_site = headers.get("sec-fetch-site")
+    if fetch_site is not None:
+        # "same-origin", "same-site" (our own subdomains), "none" (typed or
+        # bookmarked): all fine. Only another site is refused.
+        return fetch_site.strip().lower() == "cross-site"
+    origin = headers.get("origin")
+    if origin is None:
+        return False  # not a browser, or a same-origin request from an old one
+    if origin.strip().lower() == "null":
+        return True  # sandboxed frame, data: URL, file: page
+    origin_host = (urlsplit(origin.strip()).hostname or "").lower()
+    # Our own domain, whatever a reverse proxy did to the Host header.
+    domain = get_settings().app_domain.lower()
+    if origin_host == domain or origin_host.endswith("." + domain):
+        return False
+    # Otherwise the name the browser used: Host, or the one a reverse proxy saw.
+    ours = {_hostname(headers.get("host", ""))}
+    forwarded = headers.get("x-forwarded-host")
+    if forwarded:
+        ours.add(_hostname(forwarded.split(",")[0]))
+    return origin_host not in ours
+
+
+class CrossSiteGuard:
+    """ASGI middleware: 403 on cross-site POST/PUT/PATCH/DELETE under /api/."""
+
+    def __init__(self, app: ASGIApp) -> None:
+        self.app = app
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if (
+            scope["type"] == "http"
+            and scope["method"] not in _SAFE_METHODS
+            and scope["path"].startswith("/api/")
+            and is_cross_site(Headers(scope=scope))
+        ):
+            await send(
+                {
+                    "type": "http.response.start",
+                    "status": 403,
+                    "headers": [
+                        (b"content-type", b"application/json"),
+                        (b"content-length", str(len(_BODY)).encode()),
+                    ],
+                }
+            )
+            await send({"type": "http.response.body", "body": _BODY})
+            return
+        await self.app(scope, receive, send)