Email composer auto-save and crash recovery

The email composer now saves your in-progress drafts locally every 5 seconds as you type, and instantly when you add or remove recipients, change the sender, attach files, or toggle tracking. If your browser crashes, reloads, or you accidentally close a tab, your draft restores automatically when you return. Only one tab at a time can own and auto-save a draft, preventing tabs from overwriting each other. The draft clears only when you discard it, save it to Drafts, or successfully send it—failed sends never lose your text. Auto-saved data is tied to your logged-in account, cleared on logout, and removed on app load if you log into a different account on the same browser.