Implement keyboard shortcuts and accessibility baseline

## Goal
Ship an accessibility-first interaction model inspired by Desmos scientific shortcuts and settings.

## Scope
- Keyboard navigation across expression lines and within math fields
- Common shortcut support (undo/redo, clear, angle toggle)
- Fraction/navigation shortcuts in structured math input
- Accessibility settings:
- enlarged display mode
- reverse contrast mode
- braille mode scaffolding/hooks (UEB/Nemeth-ready architecture)
- Screen-reader-friendly math verbalization strategy (at minimum for focused expression and answer)

## Acceptance Criteria
- Core keyboard-only workflows can be completed without mouse.
- Reverse contrast and enlarged display can be toggled and persisted.
- Shortcut coverage is documented in-app.
- Accessibility smoke tests are added (manual checklist + automated where possible).