feat: implement InputHandler for keyboard input (Task 5) (#8)

- Extend Key enum from 20 to 115 complete USB HID keyboard codes
  - All letters A-Z, digits 0-9, punctuation, function keys F1-F24
  - Navigation keys (Home, End, Page Up/Down, Insert, Delete)
  - Arrow keys, keypad keys, international keys

- Add InputHandler class (lib/input-handler.ts - 325 lines)
  - Smart encoding strategy: direct ASCII, Ghostty encoder, VT100 sequences
  - Complete keyboard event handling with modifier extraction
  - Proper resource cleanup with dispose()

- Add comprehensive test suite (lib/input-handler.test.ts - 610 lines)
  - 34 tests covering all key types and edge cases
  - Mock DOM implementation for testing without dependencies
  - All tests passing

- Add interactive demo (examples/input-demo.html - 458 lines)
  - Real-time terminal input display with hex dump
  - Statistics tracking (keys pressed, bytes generated, ctrl combos)
  - Beautiful modern UI for visual testing

- Fix ghostty.ts setOption to handle undefined return values

Test results: 97 total tests passing (63 buffer + 34 input handler)
Task 5 complete and ready for integration into Task 6

Author: sreya