M6 hosted mailboxes (off by default); dark message mode; fixes

Hosted mailboxes (SPEC App. D), behind HOSTED_MAILBOXES (off: unchanged):
- Stalwart mail server, provisioned by `python -m app.hosted.provision`;
  directory read from a Postgres view, app passwords only for mail apps
- native account per user, the worker signs in as master for the user
- raw messages of hosted mailboxes only cached, fetched back on demand
- migrations 0025 (accounts.kind, directory view), 0026 (IMAP ids bigint)

Mail:
- dark message mode switch in the reading pane (Thunderbird's algorithm)
- moves and folder creation quote the folder name ("Deleted Items"):
  swipe-to-delete / Trash no longer bounce back
- thread unread counters: lock before recount, sorted lock order,
  UIDVALIDITY re-import recounts, nightly repair_threads
- sidebar accounts sorted alphabetically, more space above headings

Settings: Security headings renamed; antispam accounts are explicit
(none ticked = none filtered), input widths, Activity hidden.
Plugin bundles served with Cache-Control: no-cache.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/indexer/thumbs.py

@@ -87,6 +87,11 @@ def _render(data: bytes, mime_type: str) -> bytes | None:
     return out.getvalue()
 
 
+def made(user_id: uuid.UUID, sha: str) -> bool:
+    """Whether the attachment's thumbnail (or its absence) is settled already."""
+    return _path(user_id, sha, ".webp").exists() or _path(user_id, sha, ".none").exists()
+
+
 def thumbnail(
     user_id: uuid.UUID,
     sha: str,