Email warmup guidance in Compose

When you use a new mailbox, providers like Gmail and Outlook need time to build trust with your address. Webmail now displays a friendly guidance card at the top of Compose (for new messages, replies, and forwards) explaining this warmup period and linking to a 2-week warmup checklist. The card shows until you dismiss it with a "Got it" button or your email plan reaches the 30-day mark, with the dismissal preference remembered per mailbox. This helps you understand why emails might land in spam and what steps to take early on.