New feature: black and white color mode

[toolicious]

Implements the black and white mode from #150. Measurements are in the issue comment.

Adds ColorMode.BLACK_AND_WHITE, selectable per page and as a default in the settings. The binarization runs on the output of the existing grayscale pipeline and adds a Sauvola threshold. PDF export embeds these pages as 1 bit per pixel with CCITT group 4, rebuilt from the original capture. Automatic detection never selects the new mode, page geometry and the other two modes are untouched, and there is no new dependency.

Three things you may want to decide differently:

• Resolution for this mode is expressed in dpi (150 / 300 / 450) rather than as a pixel budget, because dots per inch is what decides whether 1 bit per pixel looks sharp. It reuses the physical page size the app already estimates for the PDF page box, and falls back to A4 when that estimate is not available. The highest setting interpolates beyond the capture resolution to get finer edges.
• The name, "Black & white" next to "Grayscale".
• I put the three new strings into all locale files, machine translated, the way the other string commits in the repo do it. Happy to drop those and leave you the base file only.

While measuring I also noticed two things unrelated to the feature: during export the page image was fetched twice per page, and the OCR bitmap was decoded even when no OCR language is enabled. Both are fixed in this branch, since they distorted my numbers. I can split them into a separate PR if you prefer.

Tested on a device and with ./gradlew clean license check assembleRelease.

[pynicolas]

Thanks a lot for your PR!
I will have a look at it but probably not in the next few days.