chore: resolve react-hooks/exhaustive-deps warnings

Goes through the 7 exhaustive-deps warnings individually:

Added the genuinely-missing dependency (safe, no extra churn):
- email-viewer useMemo: add effectiveEmailContent.hasStyleTag (used for
  hasOwnLayout; changes in lockstep with .html, closing a latent staleness gap).
- pro-compose-tab-body handleSend: add refreshCurrentMailbox (stable zustand
  selector) and drop the stale fetchEmails/selectedMailbox deps — which left
  those two selectors entirely unused, so remove them too.
- use-mailbox-drop handleDrop: add sourceMailboxId (changes in lockstep with
  draggedEmails, already a dep).

Suppressed with a justified comment where depending on the whole object would
regress behavior — these are intentional fine-grained deps:
- email-composer signature-swap effect (keyed to signature fields + prev*Ref
  guards; whole signatureIdentity would re-splice the live editor).
- email-viewer auto-mark-as-read (whole email would reset the delay timer on
  any unrelated field update).
- email-viewer effective-attachments memo (derives from email.attachments;
  whole email would churn the list + its layout measurement).
- email-viewer auto-MDN effect (email already captured via id +
  sendReadReceiptNow; autoMdnRef guards double-send).

tsc --noEmit clean; eslint now reports 0 problems.

components/pro/pro-compose-tab-body.tsx

@@ -25,11 +25,9 @@ export function ProComposeTabBody({ tabId, data }: ProComposeTabBodyProps) {
   const t = useTranslations();
   const client = useAuthStore((s) => s.client);
   const sendEmail = useEmailStore((s) => s.sendEmail);
-  const fetchEmails = useEmailStore((s) => s.fetchEmails);
   const refreshCurrentMailbox = useEmailStore((s) => s.refreshCurrentMailbox);
   const fetchScheduledEmails = useEmailStore((s) => s.fetchScheduledEmails);
   const refreshScheduledMetadata = useEmailStore((s) => s.refreshScheduledMetadata);
-  const selectedMailbox = useEmailStore((s) => s.selectedMailbox);
   const isScheduledView = useEmailStore((s) => s.isScheduledView);
   const closeTab = useProTabStore((s) => s.closeTab);
   const updateTabTitle = useProTabStore((s) => s.updateTabTitle);
@@ -116,7 +114,7 @@ export function ProComposeTabBody({ tabId, data }: ProComposeTabBodyProps) {
       console.error('Failed to send email:', error);
       toast.error(t('notifications.error_sending'));
     }
-  }, [client, sendEmail, fetchEmails, selectedMailbox, closeTab, data.sourceEmailId, data.mode, t, handleScheduledSendCreated]);
+  }, [client, sendEmail, closeTab, data.sourceEmailId, data.mode, t, handleScheduledSendCreated, refreshCurrentMailbox]);
 
   const handleClose = useCallback(() => {
     closeTab(tabIdRef.current);