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>

plugins/webmail_plugin_hello/webmail_plugin_hello/router.py

@@ -52,4 +52,8 @@ async def add_note(body: NoteIn, user_id: CurrentUserId) -> dict[str, str]:
 @router.get("/ui/page.js")
 async def page_bundle() -> FileResponse:
     """Serve the page's ESM bundle for the frontend loader (SPEC §11.3)."""
-    return FileResponse(_UI / "page.js", media_type="text/javascript")
+    # `no-cache`: revalidated on every load (a cheap 304 via the ETag), so an
+    # updated bundle is used at once instead of a copy the browser kept.
+    return FileResponse(
+        _UI / "page.js", media_type="text/javascript", headers={"Cache-Control": "no-cache"}
+    )