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/provider_factory.py

@@ -3,6 +3,9 @@
 This module runs only inside the worker/sync context; ``decrypt_secret`` (the
 credential vault) is invoked here to obtain the plaintext mailbox password. It
 must never be imported by API request code.
+
+A native account (the user's hosted mailbox, App. D §D.4) has no stored
+password: the worker signs in as itself, acting for the user.
 """
 
 from __future__ import annotations
@@ -14,6 +17,10 @@ from app.security.crypto import decrypt_secret
 
 def build_provider(account: Account) -> ImapSmtpProvider:
     """Construct an :class:`ImapSmtpProvider` for ``account`` (password auth, M0)."""
+    if account.kind == "native":
+        from app.hosted.native import native_provider_config
+
+        return ImapSmtpProvider(native_provider_config(account.email))
     imap = account.imap_config
     smtp = account.smtp_config
     password = decrypt_secret(account.secret_ct).decode("utf-8")