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/mail/test_actions.py

@@ -421,3 +421,36 @@ async def test_delete_keeps_rows_while_unreachable(ctx: Ctx, monkeypatch) -> Non
             select(func.count()).select_from(Message).where(Message.thread_id == ctx.thread_id)
         )
         assert left == len(ctx.message_ids)  # kept: the server still has them
+
+
+async def test_restore_moves_only_what_was_removed(ctx: Ctx, monkeypatch) -> None:  # type: ignore[no-untyped-def]
+    """A thread restored from Spam brings its spam back; the user's own reply in
+    Sent (never removed, so no recorded origin) stays in Sent."""
+    sent = Mailbox(account_id=None, user_id=ctx.user_id, path="Sent", special_use="sent")
+    first = await ctx.session.get(Message, ctx.message_ids[0])
+    reply = await ctx.session.get(Message, ctx.message_ids[1])
+    assert first is not None and reply is not None
+    sent.account_id = first.account_id
+    ctx.session.add(sent)
+    await ctx.session.flush()
+    first.mailbox_id = ctx.archive_id  # removed, origin unknown (as when synced in)
+    reply.mailbox_id = sent.id
+    await ctx.session.commit()
+
+    moves: list[tuple[int, str, str]] = []
+
+    class Recorder:
+        async def move(self, uid: int, src: str, dst: str) -> int:
+            moves.append((uid, src, dst))
+            return 100 + uid
+
+        async def close(self) -> None:
+            return None
+
+    monkeypatch.setattr(sync_actions, "build_provider", lambda _a: Recorder())
+    await sync_actions.restore_thread_job({"job_try": 1}, str(ctx.thread_id))
+    assert moves == [(first.uid, "Archive", "INBOX")]
+    ctx.session.expunge_all()
+    async with session_scope() as verify:
+        kept = await verify.get(Message, ctx.message_ids[1])
+        assert kept is not None and kept.mailbox_id == sent.id