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/migrations/versions/0026_imap_ids_bigint.py

@@ -0,0 +1,40 @@
+"""IMAP ids as 64-bit integers
+
+UIDVALIDITY and UIDs are unsigned 32-bit numbers (RFC 9051), MODSEQ up to 63
+bits (RFC 7162); Postgres ``integer`` is signed 32-bit. Servers that hand out
+values above 2^31 — the hosted mail server's UIDVALIDITY, for one — could not
+be stored ("value out of int32 range") and their folders never synced.
+
+Revision ID: 0026
+Revises: 0025
+Create Date: 2026-10-11
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "0026"
+down_revision: str | None = "0025"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+_COLUMNS = (
+    ("mailboxes", "uidvalidity"),
+    ("mailboxes", "highestmodseq"),
+    ("mailboxes", "last_seen_uid"),
+    ("messages", "uid"),
+    ("drafts", "imap_uid"),
+)
+
+
+def upgrade() -> None:
+    for table, column in _COLUMNS:
+        op.execute(f"ALTER TABLE {table} ALTER COLUMN {column} TYPE bigint")
+
+
+def downgrade() -> None:
+    for table, column in _COLUMNS:
+        op.execute(f"ALTER TABLE {table} ALTER COLUMN {column} TYPE integer")