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/sync/actions.py

@@ -77,7 +77,7 @@ async def apply_flags_to_messages(
         if message.thread_id is not None:
             thread_ids.add(message.thread_id)
     await session.flush()
-    for tid in thread_ids:
+    for tid in sorted(thread_ids):  # one lock order across transactions
         await denormalize_thread(session, tid)
     return thread_ids
 
@@ -504,7 +504,7 @@ async def empty_folder_job(ctx: dict[str, Any], user_id: str, special_use: str)
         await session.flush()
         # Threads with no messages left get dropped (drafts un-linked first); the
         # rest are re-denormalized so their counts/snippet reflect the removal.
-        for tid in thread_ids:
+        for tid in sorted(thread_ids):
             remaining = await session.scalar(
                 select(func.count()).select_from(Message).where(Message.thread_id == tid)
             )
@@ -584,7 +584,7 @@ async def autopurge_trash_spam(ctx: dict[str, Any]) -> None:
         )
         await session.execute(delete(Message).where(Message.id.in_(msg_ids)))
         await session.flush()
-        for tid in thread_ids:
+        for tid in sorted(thread_ids):
             remaining = await session.scalar(
                 select(func.count()).select_from(Message).where(Message.thread_id == tid)
             )