Clicking an email now opens a collapsible, horizontally resizable
reading pane beside the list instead of a new tab. Extracts the shared
EmailDetail component (full detail fetch + lazy AI summary + Open in
Gmail) and adds list skeleton + empty states via the ui primitives.
- New components/EmailDetail.jsx (emailId/email + onClose)
- New split.css (flex master-detail, native resize, <768px overlay)
- FolderView/SearchResults: selectedEmail state, onOpen wiring,
Skeleton loading rows, EmptyState zero-states
- main.jsx imports split.css
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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