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/api/messages.py

@@ -88,6 +88,23 @@ def _wrap(fragment: str) -> str:
     return _DOC_TEMPLATE.format(body=fragment)
 
 
+# The sanitized message as a document of its own. The app shows it in a
+# sandboxed frame (frontend MessageBody), but this URL can also be opened
+# directly in a tab, on our own origin: there, this policy is what keeps any
+# script the sanitizer might ever miss from running — no scripts, no plugins,
+# no forms, a sandbox of its own origin, and nothing framing it elsewhere.
+_MESSAGE_CSP = (
+    "default-src 'none'; script-src 'none'; object-src 'none'; base-uri 'none'; "
+    "form-action 'none'; frame-ancestors 'self'; img-src 'self' data: https: http:; "
+    "style-src 'self' 'unsafe-inline'; font-src 'self' data: https:; media-src 'self'; "
+    "sandbox allow-popups allow-popups-to-escape-sandbox"
+)
+
+
+def _html(document: str) -> HTMLResponse:
+    return HTMLResponse(content=document, headers={"Content-Security-Policy": _MESSAGE_CSP})
+
+
 async def _owned_message(db: AsyncSession, user_id: uuid.UUID, message_id: uuid.UUID) -> Message:
     msg = await db.scalar(
         select(Message).where(Message.id == message_id, Message.user_id == user_id)
@@ -123,7 +140,7 @@ async def message_html(
 
     stale = html_part is not None and html_part.sanitizer_v < SANITIZER_VERSION
     if html_part is not None and not allow_remote and not stale:
-        return HTMLResponse(content=_wrap(html_part.content))
+        return _html(_wrap(html_part.content))
 
     # Re-sanitize the original HTML (image-approved sender, or a stale stored copy).
     # If the raw blob is unavailable, fall back to whatever is already stored.
@@ -142,13 +159,13 @@ async def message_html(
                     html_part.content = result.html  # persist the upgraded sanitization
                     html_part.sanitizer_v = result.sanitizer_v
                     await db.commit()
-                return HTMLResponse(content=_wrap(result.html))
+                return _html(_wrap(result.html))
 
     if html_part is not None:
-        return HTMLResponse(content=_wrap(html_part.content))
+        return _html(_wrap(html_part.content))
     text_part = next((p for p in parts if p.kind == "text"), None)
     body = html_lib.escape(text_part.content) if text_part else ""
-    return HTMLResponse(content=_wrap(f'<div class="wm-plaintext">{body}</div>'))
+    return _html(_wrap(f'<div class="wm-plaintext">{body}</div>'))
 
 
 @router.post("/{message_id}/allow-remote-images", dependencies=[Depends(require_csrf)])