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_api.py

@@ -115,7 +115,15 @@ async def test_settings_defaults_then_merge(ctx: _Ctx) -> None:
     s = (await ctx.client_a.get(f"{BASE}/settings")).json()
     assert (s["move_threshold"], s["banner_threshold"]) == (12.0, 6.0)
     assert s["network_checks"] is False  # privacy default (§C.2)
-    assert s["enabled_accounts"] == []  # empty = every account
+    assert s["enabled_accounts"] is None  # every account
+
+    # Every account off is stored as such (not read back as "all on"), and
+    # all_accounts goes back to every account.
+    s = (await ctx.client_a.patch(f"{BASE}/settings", json={"enabled_accounts": []})).json()
+    assert s["enabled_accounts"] == []
+    assert (await ctx.client_a.get(f"{BASE}/settings")).json()["enabled_accounts"] == []
+    s = (await ctx.client_a.patch(f"{BASE}/settings", json={"all_accounts": True})).json()
+    assert s["enabled_accounts"] is None
 
     # PATCH merges: the field we leave out keeps its value.
     s = (await ctx.client_a.patch(f"{BASE}/settings", json={"move_threshold": 15.0})).json()