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/app/api/admin.py

@@ -19,6 +19,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.api.errors import raise_api_error
 from app.db import get_session
+from app.hosted.admin import MailServerError
 from app.identity.deps import CurrentAdmin, require_csrf
 from app.identity.mailer import LOGO_HTML, MailerError, mailer_config, send_mail
 from app.identity.schemas import (
@@ -183,7 +184,12 @@ async def patch_user(
     if body.quota_bytes is not None:
         user.quota_bytes = body.quota_bytes
     if body.blocked is not None:
-        await set_blocked(db, user, body.blocked)
+        try:
+            await set_blocked(db, user, body.blocked)
+        except MailServerError as exc:
+            raise_api_error(
+                502, "mail_server_unavailable", f"the mail server did not answer: {exc}"
+            )
     await db.commit()
     used = await db.scalar(
         select(func.coalesce(func.sum(Message.size_bytes), 0)).where(Message.user_id == user.id)
@@ -200,5 +206,8 @@ async def patch_user(
 async def remove_user(user_id: uuid.UUID, admin: CurrentAdmin, db: DbSession) -> dict[str, str]:
     """Delete a user and all of their data. Cannot be undone."""
     user = await _target(db, admin, user_id)
-    await delete_user(db, user)
+    try:
+        await delete_user(db, user)
+    except MailServerError as exc:
+        raise_api_error(502, "mail_server_unavailable", f"the mail server did not answer: {exc}")
     return {"status": "deleted"}