P0 Core:
- Sandboxed iframe rendering for rich HTML emails (CSS isolation)
- API retry with exponential backoff for transient JMAP failures
- Mobile bottom action bar with touch-friendly email actions
- Long-press context menu + tap-to-expand submenus on touch devices
P1 Polish:
- Tag counts sidebar section with JMAP batch queries
- Empty folder option for Junk/Trash with batch delete
- Extra-compact density option (28px rows, 44px on touch)
- SPF/DKIM/DMARC security tooltips with plain-language explanations
- Resizable sidebars with drag, touch, and keyboard support
- Expandable sender info panel in email viewer
All 8 locales updated (en/fr/ja/es/it/de/nl/pt).
