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/conftest.py

@@ -18,6 +18,19 @@ os.environ.setdefault(
 os.environ["REDIS_URL"] = "redis://127.0.0.1:6380/15"
 os.environ.setdefault("MASTER_KEY", base64.b64encode(b"\x00" * 32).decode())
 os.environ.setdefault("BLOB_ROOT", "./var/blobs-test")
+# Hosted mailboxes off: tests that need the mail server turn them on themselves
+# (tests/hosted/), against the dev stack's Stalwart container.
+os.environ["HOSTED_MAILBOXES"] = "false"
+for _key, _value in {
+    "STALWART_URL": "http://127.0.0.1:8180",
+    "STALWART_ADMIN_SECRET": "stalwart_admin_dev",
+    "STALWART_MASTER_SECRET": "stalwart_master_dev",
+    "STALWART_IMAP_HOST": "127.0.0.1",
+    "STALWART_IMAP_PORT": "1993",
+    "STALWART_SMTP_HOST": "127.0.0.1",
+    "STALWART_SMTP_PORT": "1465",
+}.items():
+    os.environ.setdefault(_key, _value)
 
 from collections.abc import AsyncIterator  # noqa: E402