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.

hooks/use-mailbox-drop.ts

@@ -222,7 +222,7 @@ export function useMailboxDrop({ mailbox, onDropComplete, onSuccess, onError }:
     } finally {
       endDrag();
     }
-  }, [client, mailbox, mailboxes, isValidTarget, moveEmailsToMailbox, crossAccountMoveEmails, draggedEmails, selectedEmailIds, clearSelection, refreshCurrentMailbox, endDrag, onDropComplete, onSuccess, onError]);
+  }, [client, mailbox, mailboxes, isValidTarget, moveEmailsToMailbox, crossAccountMoveEmails, draggedEmails, sourceMailboxId, selectedEmailIds, clearSelection, refreshCurrentMailbox, endDrag, onDropComplete, onSuccess, onError]);
 
   const valid = isValidTarget();