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/tests/plugins/test_antispam_learning.py

@@ -14,7 +14,7 @@ import pytest
 from webmail_plugin_antispam import hook, jobs, policy
 
 SETTINGS = policy.Settings(
-    enabled_accounts=[],
+    enabled_accounts=None,
     move_threshold=12.0,
     banner_threshold=6.0,
     network_checks=False,
@@ -169,7 +169,7 @@ async def test_bootstrap_respects_the_setting(monkeypatch: pytest.MonkeyPatch) -
 
     async def _settings(_u: object) -> policy.Settings:
         return policy.Settings(
-            enabled_accounts=[],
+            enabled_accounts=None,
             move_threshold=12.0,
             banner_threshold=6.0,
             network_checks=False,