fix: ribbon tabs now show per-tab content; add all missing handleRibbonAction handlers
- RibbonBar: conditional rendering per activeTab (start/insert/format/view/tools/ki)
- App.tsx: add onNavigateBack prop for new/open navigation
- App.tsx: implement handlers for new, open, save, copy, paste, zoom-fit, zoom-100, grid, layer, measure, search, plugins, ki, ki-draw, ki-analyze
- App.tsx: implement insert-line, insert-rect, insert-circle, insert-text, insert-freehand, insert-image
- App.tsx: implement format-* placeholder actions
- App.tsx: add clipboard state for copy/paste
- All 343 tests pass, tsc clean
Leopoldadmin
3bec3938d0