add output format HTML

[milahu]

fix #6 (comment)

example output with grounded olmocr
2026-03-26.22-00.olmocr.html

this is a minimal-diff patch, hooking into the PDF writer module
ideally this should be a separate module in src/pdf_ocr/core/html.py
but that requires patching some call sites
for examlpe by replacing the pdf_handler kwarg
with input_handler and output_handler
or src_handler and dst_handler

use letter-spacing to stretch text to line width
this also works on span elements with display:inline
transform:scaleX would require display:block

[milahu]

closing in favor of #10