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/sync/test_secondary_poll.py

@@ -371,3 +371,68 @@ async def test_new_inbox_mail_while_live_asks_for_a_push(monkeypatch) -> None:
             await cleanup.execute(delete(User).where(User.id == user.id))
         await session.close()
         await engine.dispose()
+
+
+async def test_live_loop_catches_up_before_it_idles(monkeypatch) -> None:  # type: ignore[no-untyped-def]
+    """Mail that reached INBOX while the loop was not running (a new account's
+    first minutes, a reconnect) is fetched at once, not after the next new
+    message or the 10-minute IDLE timeout."""
+    import asyncio
+
+    engine = create_async_engine(get_settings().database_url, poolclass=NullPool)
+    sm = async_sessionmaker(engine, expire_on_commit=False)
+    session = sm()
+    user = User(handle=f"idl{uuid.uuid4().hex[:8]}@test.local", password_hash=hash_password("pw"))
+    session.add(user)
+    await session.flush()
+    account = Account(
+        user_id=user.id,
+        email="me@example.com",
+        imap_config={},
+        smtp_config={},
+        secret_ct=b"",
+        discovery_src="manual",
+    )
+    session.add(account)
+    await session.flush()
+    inbox = Mailbox(account_id=account.id, user_id=user.id, path="INBOX", special_use="inbox")
+    session.add(inbox)
+    await session.commit()
+
+    calls: list[str] = []
+    stop = asyncio.Event()
+
+    class FakeProvider:
+        async def verify(self) -> None:
+            return None
+
+        async def poll(self, mailbox: str, *, last_seen_uid: int, highestmodseq: int | None):  # type: ignore[no-untyped-def]
+            calls.append("poll")
+            waiting = RemoteMessage(
+                uid=1,
+                raw=_raw("Arrived while away", "<away@x.com>"),
+                flags=frozenset(),
+                internal_date=datetime.now(UTC),
+            )
+            return MailboxDelta([waiting] if len(calls) == 1 else [], [], [], None, None)
+
+        async def idle(self, mailbox: str) -> None:
+            calls.append("idle")
+            stop.set()
+
+        async def close(self) -> None:
+            return None
+
+    monkeypatch.setattr(sync_engine, "build_provider", lambda _a: FakeProvider())
+    try:
+        await asyncio.wait_for(sync_engine.live_loop(account.id, "INBOX", stop), 10)
+        assert calls == ["poll", "idle"]
+        n = await session.scalar(
+            select(func.count()).select_from(Message).where(Message.mailbox_id == inbox.id)
+        )
+        assert n == 1
+    finally:
+        async with session_scope() as cleanup:
+            await cleanup.execute(delete(User).where(User.id == user.id))
+        await session.close()
+        await engine.dispose()