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>

docker/stalwart/config.json

@@ -0,0 +1,15 @@
+{
+  "@type": "PostgreSql",
+  "host": "postgres",
+  "port": 5432,
+  "database": "stalwart",
+  "authUsername": "stalwart",
+  "authSecret": { "@type": "EnvironmentVariable", "variableName": "STALWART_DB_PASSWORD" },
+  "useTls": false,
+  "allowInvalidCerts": false,
+  "poolMaxConnections": 10,
+  "poolRecyclingMethod": "fast",
+  "timeout": 15000,
+  "readReplicas": {},
+  "options": null
+}