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>

frontend/src/test/ThreadView.test.tsx

@@ -5,6 +5,7 @@ import { describe, expect, it } from "vitest";
 
 import { ThreadView } from "../components/thread/ThreadView";
 import { I18nProvider } from "../i18n";
+import { ThemeProvider } from "../lib/theme";
 import type { MessageEnvelope } from "../lib/types";
 
 function msg(id: string, minutes: number, unread = false): MessageEnvelope {
@@ -38,7 +39,9 @@ function wrap(children: ReactNode) {
   const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
   return (
     <I18nProvider>
-      <QueryClientProvider client={qc}>{children}</QueryClientProvider>
+      <ThemeProvider>
+        <QueryClientProvider client={qc}>{children}</QueryClientProvider>
+      </ThemeProvider>
     </I18nProvider>
   );
 }