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/sync/actions.py

@@ -302,7 +302,8 @@ async def restore_thread_job(ctx: dict[str, Any], thread_id: str) -> None:
 
     Each message returns to ``restore_mailbox_id`` (the folder it was in before it
     was archived/trashed) — e.g. a deleted Sent message goes back to Sent. Messages
-    with no recorded origin fall back to the account's Inbox.
+    with no recorded origin fall back to the account's Inbox. Only messages in
+    Archive, Trash or Spam move: the thread's others (your reply in Sent) stay.
     """
     tid = uuid.UUID(thread_id)
     async with session_scope() as session:
@@ -319,10 +320,13 @@ async def restore_thread_job(ctx: dict[str, Any], thread_id: str) -> None:
             await session.scalars(select(Mailbox).where(Mailbox.account_id.in_(account_ids)))
         )
         path_by_id = {mb.id: mb.path for mb in mailboxes}
+        removal_ids = {mb.id for mb in mailboxes if mb.special_use in _REMOVAL_FOLDERS}
         inbox_by_account = {mb.account_id: mb.id for mb in mailboxes if mb.special_use == "inbox"}
         # (message_id, account_id, source_path, target_id) — resolve the target now.
         plan: list[tuple[uuid.UUID, uuid.UUID, str, uuid.UUID, int]] = []
         for m in messages:
+            if m.mailbox_id not in removal_ids:
+                continue  # never removed: nothing to restore
             target_id = m.restore_mailbox_id
             if target_id is None or target_id not in path_by_id:
                 target_id = inbox_by_account.get(m.account_id)