feat(pdftract-2wqir): implement keyboard shortcuts in inspector
Added comprehensive keyboard shortcuts for the inspector frontend:
- ArrowLeft/Right: navigate to previous/next page
- ArrowUp/Down: scroll within page
- /: focus search input
- Esc: blur input / close help overlay
- ?: show/hide keyboard shortcuts help overlay
- 1-9: toggle overlay layers (1=spans, 2=blocks, ..., 9=diff)
Changes:
- app.js: extended setupKeyboard() with new handlers, added prevPage()/nextPage() wrappers, scrollPage() and toggleHelp() helpers, setupHelp() for button wiring
- index.html: added ? button and help overlay with all shortcuts listed
- style.css: added styles for .btn-help, .help-overlay, .help-content, and related classes
Acceptance criteria met:
- ArrowLeft/Right navigation works
- / focuses search input
- 1-8 toggle overlays with visual feedback
- Esc blurs input and closes help
- ? shows help overlay listing all shortcuts
See: notes/pdftract-2wqir.md for verification details.
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