feat(inbox): two-pane layout with empty state overlays

- Resizable left list pane + right detail pane when reports exist
- Full-width view with skeleton backdrop + gradient overlay for empty states
- Welcome state (graphs-hog), warming-up (explorer-hog), select-a-report (mail-hog)
- Inline signal source config modal instead of Settings navigation
- "Back to Inbox" button disabled with tooltip until sources enabled
- Loading skeleton only when sources are configured
- Search disabled with tooltip when no reports

Author: Twixes