Fix dark-mode email readability, silent reply failures, and add an IMAP connection watchdog

Dark theme: inbound email HTML intentionally keeps its own inline colors
for fidelity (sanitize-html.ts), but a sender's hardcoded dark text (the
common case) was unreadable once the surrounding chrome went dark. Now
forced to the theme's own text color in dark mode only; light mode keeps
full fidelity as before.

Send button: deliverAgentMessage awaited the outbound email/Telegram send
*before* recording the reply, so a broken mail/bot account (e.g. bad SMTP
creds) made the reply vanish entirely — not sent, not saved, no error
shown — with the API's catch-all masking the real cause as a generic
"Ticket not found". Channel delivery is now best-effort: the reply is
always recorded even if outbound delivery fails, and real errors surface
to the compose box instead of failing silently.

IMAP watchdog: a live IDLE connection has gone quietly unresponsive in
production more than once, with mail landing server-side but never
noticed here and no error ever firing. Added a periodic forced NOOP
round-trip with its own deadline, independent of ImapFlow's internal
socket timeout, that tears down and reconnects a connection that stops
answering.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gh2UXUQUVBWroWEnn1FLFG

src/components/message-body-html.tsx

@@ -5,7 +5,7 @@
 export function MessageBodyHtml({ html, className = "" }: { html: string; className?: string }) {
   return (
     <div
-      className={`max-w-full overflow-x-auto text-sm break-words [&_img]:max-w-full [&_img]:cursor-zoom-in [&_table]:max-w-full [&_td]:break-words [&_th]:break-words ${className}`}
+      className={`message-html max-w-full overflow-x-auto text-sm break-words [&_img]:max-w-full [&_img]:cursor-zoom-in [&_table]:max-w-full [&_td]:break-words [&_th]:break-words ${className}`}
       dangerouslySetInnerHTML={{ __html: html }}
     />
   );