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_policy.py

@@ -6,7 +6,7 @@ import pytest
 from webmail_plugin_antispam.policy import Settings, decide, matches
 
 SETTINGS = Settings(
-    enabled_accounts=[],
+    enabled_accounts=None,
     move_threshold=12.0,
     banner_threshold=6.0,
     network_checks=False,
@@ -79,7 +79,7 @@ def test_pattern_matching(patterns: set[str], email: str, hit: bool) -> None:
 
 
 def test_per_account_enablement() -> None:
-    """Empty list means every account; a populated one is an allow-list."""
+    """None means every account; a list is an allow-list — empty, none."""
     everywhere = SETTINGS
     assert everywhere.covers("any-account-id")
 
@@ -93,6 +93,15 @@ def test_per_account_enablement() -> None:
     assert scoped.covers("acc-1")
     assert not scoped.covers("acc-2")
 
+    nowhere = Settings(
+        enabled_accounts=[],
+        move_threshold=12.0,
+        banner_threshold=6.0,
+        network_checks=False,
+        learn_from_provider_junk=True,
+    )
+    assert not nowhere.covers("acc-1")
+
 
 # --- contacts are never filed as spam ----------------------------------------